Re: [bitcoin-dev] BIP proposal: NODE_NETWORK_LIMITED service bits

From: Gregory Maxwell <greg@xiph•org>
To: Bitcoin Dev <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP proposal: NODE_NETWORK_LIMITED service bits
Date: Thu, 11 May 2017 18:17:19 +0000	[thread overview]
Message-ID: <CAAS2fgS01Saxb8AtZhSxVGH5XFmXDzaaB0gd+4Zhr3-ahUJbfQ@mail.gmail.com> (raw)
In-Reply-To: <E1313B4E-6061-49CA-9E8C-E5FD468531C0@jonasschnelli.ch>

It probably should be stated in terms of what you're promising to do--
 288 and 1152 blocks, not what we hope it will accomplish. Then advise
clients to use peers with headroom because their estimates could be
wrong and reorgs.

Reorgs aren't the only concerns that drive larger numbers:  The peak
at syncing is at ~24 hours, but sometimes there are quite a few more
than 144 blocks in 24 hours. Also, new blocks show up in the chain:
you think you're 144 behind but by the time you connect you find
you're 146 behind from that peer's perspective.

I think it's a bit ambiguous what it's saying about the headers,
especially because it goes into detail about address relay. I believe
nodes with any of these settings should be willing to serve headers
for their entire best chain.  Perhaps you could say that this is
equivalent to NODE_NETWORK except that they aren't necessarily willing
to server historical blocks.

I'm unsure about the third depth level. Perhaps that should be left
undefined for sending right now and treated as least 1152 blocks by
receivers-- I don't have any reason to think 7056 is a particularly
useful choice, and we'll need another (longer) level for UTXO based
sync.   You could probably go further and say that nodes shouldn't
send it now, but if sent it means they intend to keep 2016*2 blocks.
(Not sending because the requirement for sending it may be that the
node is able to send you a UTXO data feed.)

> consider to switch a low percentage

That isn't grammatical, you want "switching".  But I think it would be
better to say that when a node believe it is in sync enough to use
NODE_NETWORK_LIMITED_X it should just treat them identically to
NODE_NETWORK in peer selection.   We don't really need any more
topology distortion than that.   In particular, I don't want to be in
a case where NODE_NETWORK peers suddenly find themselves far less well
connected.

In terms of making room, a node network peer could choose to
disconnect the least useful peers that aren't syncing from them to
make more room for ones that are.  This lets them decide what
connections they want, based on how full they are and what is useful
to them, rather than finding themselves all lonely because nodes
decided to avoid them to be "helpful", and we already use
disconnections to manage fullness.

On Thu, May 11, 2017 at 3:13 PM, Jonas Schnelli via bitcoin-dev
<bitcoin-dev@lists•linuxfoundation.org> wrote:
> Hi
>
> Currently, pruned peers have no way how to signal their (valuable) service.
> A BIP proposal to improve this (draft):
> https://github.com/jonasschnelli/bips/wiki/NODE_NETWORK_LIMITED-BIP-DRAFT
>
> Feedback is highly welcome.
>
> </jonas>
>
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