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From: Suhas Daftuar <sdaftuar@gmail.com>
Date: Mon, 17 Aug 2020 16:40:02 -0400
Message-ID: <CAFp6fsH==PJ38vGvCGUr3AVS8XmQEjvLkNaopAhu0vY7xQjgAw@mail.gmail.com>
To: Eric Voskuil <eric@voskuil.org>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Generalizing feature negotiation when new p2p
 connections are setup
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Hi Eric,

Thanks for your response.  If I understand correctly, you're suggesting
that in the future we do the same as what was done in BIP 339, of
accompanying new messages (which are optional) with a protocol version
bump, so that network clients are never reading unknown messages from a
peer (and can be free to disconnect a peer for sending an unknown message)?

I think that works fine, so if indeed there will be software that will
expect things to operate this way then I can withdraw the suggestion I've
made in this thread.  However I wanted to clarify that this is what you
suggest, because there is another downside to this approach (beyond the
sequential nature of sequence numbers that you mention) -- if a software
implementation misses a proposed new protocol upgrade, and thus fails to
parse (and ignore) some proposed new message, the result can be a network
split down the road as incompatible clients get slowly upgraded over time.

I think this coordination cost is something to be concerned about -- for
instance, the lack of response to my wtxid-relay proposal made me wonder if
other software would be implementing something to account for the new
message that proposal introduces (for clients with version >=3D 70016).  It=
's
reasonable for people to be busy and miss things like this, and I think
it's worth considering whether there's a safer way for us to deploy changes=
.

That said, I don't think this coordination cost is unbearable, so as long
as we have a process for making p2p protocol improvements I'm not too
worried about what mechanism we use.  So if this concern over coordination
of changes doesn't sway you, I think we can continue to just bump protocol
version at the same time as deploying new messages, as we have been doing,
and hope that we don't run into problems down the road.

If I have misunderstood how you think we should be making future protocol
changes, please let me know.

Thanks,
Suhas



On Sun, Aug 16, 2020 at 3:06 PM Eric Voskuil <eric@voskuil.org> wrote:

> A requirement to ignore unknown (invalid) messages is not only a protocol
> breaking change but poor protocol design. The purpose of version
> negotiation is to determine the set of valid messages. Changes to version
> negotiation itself are very problematic.
>
> The only limitation presented by versioning is that the system is
> sequential. As such, clients that do not wish to implement (or operators
> who do not wish to enable) them are faced with a problem when wanting to
> support later features. This is resolvable by making such features option=
al
> at the new protocol level. This allows each client to limit its
> communication to the negotiated protocol, and allows ignoring of known bu=
t
> unsupported/disabled features.
>
> Sorry I missed your earlier post. Been distracted for a while.
>
> e
>
>
> On Aug 14, 2020, at 12:28, Suhas Daftuar via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> =EF=BB=BF
> Hi,
>
> Back in February I posted a proposal for WTXID-based transaction relay[1]
> (now known as BIP 339), which included a proposal for feature negotiation
> to take place prior to the VERACK message being received by each side.  I=
n
> my email to this list, I had asked for feedback as to whether that propos=
al
> was problematic, and didn't receive any responses.
>
> Since then, the implementation of BIP 339 has been merged into Bitcoin
> Core, though it has not yet been released.
>
> In thinking about the mechanism used there, I thought it would be helpful
> to codify in a BIP the idea that Bitcoin network clients should ignore
> unknown messages received before a VERACK.  A draft of my proposal is
> available here[2].
>
> I presume that software upgrading past protocol version 70016 was already
> planning to either implement BIP 339, or ignore the wtxidrelay message
> proposed in BIP 339 (if not, then this would create network split concern=
s
> in the future -- so I hope that someone would speak up if this were a
> problem).  When we propose future protocol upgrades that would benefit fr=
om
> feature negotiation at the time of connection, I think it would be nice t=
o
> be able to use the same method as proposed in BIP 339, without even needi=
ng
> to bump the protocol version.  So having an understanding that this is th=
e
> standard of how other network clients operate would be helpful.
>
> If, on the other hand, this is problematic for some reason, I look forwar=
d
> to hearing that as well, so that we can be careful about how we deploy
> future p2p changes to avoid disruption.
>
> Thanks,
> Suhas Daftuar
>
> [1]
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-February/017=
648.html
>
> [2]
> https://github.com/sdaftuar/bips/blob/2020-08-generalized-feature-negotia=
tion/bip-p2p-feature-negotiation.mediawiki
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>

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<div dir=3D"ltr">Hi Eric,<div><br></div><div>Thanks for your response.=C2=
=A0 If I understand correctly, you&#39;re suggesting that in the future we =
do the same as what was done in BIP 339, of accompanying new messages (whic=
h are optional) with a protocol version bump, so that network clients are n=
ever reading unknown messages from a peer (and can be free to disconnect a =
peer for sending an unknown message)?</div><div><br></div><div>I think that=
 works fine, so if indeed there will be software that will expect things to=
 operate this way then I can withdraw the suggestion I&#39;ve made in this =
thread.=C2=A0 However I wanted to clarify that this is what you suggest, be=
cause there is another downside to this approach (beyond the sequential nat=
ure of sequence numbers that you mention) -- if a software implementation m=
isses a proposed new protocol upgrade, and thus fails to parse (and ignore)=
 some proposed new message, the result can be a network split down the road=
 as incompatible clients get slowly upgraded over time.=C2=A0</div><div><br=
></div><div>I think this coordination cost is something to be concerned abo=
ut -- for instance, the lack of response to my wtxid-relay proposal made me=
 wonder if other software would be implementing something to account for th=
e new message that proposal introduces (for clients with version &gt;=3D 70=
016).=C2=A0 It&#39;s reasonable for people to be busy and miss things like =
this, and I think it&#39;s worth considering whether there&#39;s a safer wa=
y for us to deploy changes.</div><div><br></div><div>That said, I don&#39;t=
 think this coordination cost is unbearable, so as long as we have a proces=
s for making p2p protocol improvements I&#39;m not too worried about what m=
echanism we use.=C2=A0 So if this concern over coordination of changes does=
n&#39;t sway you, I think we can continue to just bump protocol version at =
the same time as deploying new messages, as we have been doing, and hope th=
at we don&#39;t run into problems down the road.=C2=A0=C2=A0</div><div><br>=
</div><div>If I have misunderstood how you think we should be making future=
 protocol changes, please let me know.</div><div><br></div><div>Thanks,</di=
v><div>Suhas</div><div><br></div><div><br></div></div><br><div class=3D"gma=
il_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Sun, Aug 16, 2020 at 3:0=
6 PM Eric Voskuil &lt;<a href=3D"mailto:eric@voskuil.org" target=3D"_blank"=
>eric@voskuil.org</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote"=
 style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);p=
adding-left:1ex"><div dir=3D"auto"><div dir=3D"ltr"><div dir=3D"ltr">A requ=
irement to ignore unknown (invalid) messages is not only a protocol breakin=
g change but poor protocol design. The purpose of version negotiation is to=
 determine the set of valid messages. Changes to version negotiation itself=
 are very problematic.</div><div dir=3D"ltr"><br></div><div dir=3D"ltr">The=
 only limitation presented by versioning is that the system is sequential. =
As such, clients that do not wish to implement (or operators who do not wis=
h to enable) them are faced with a problem when wanting to support later fe=
atures. This is resolvable by making such features optional at the new prot=
ocol level. This allows each client to limit its communication to the negot=
iated protocol, and allows ignoring of known but unsupported/disabled featu=
res.</div><div dir=3D"ltr"><br></div><div dir=3D"ltr">Sorry I missed your e=
arlier post. Been distracted for a while.</div><div dir=3D"ltr"><br></div><=
div dir=3D"ltr">e</div><div dir=3D"ltr"><br></div><div dir=3D"ltr"><br></di=
v></div><div dir=3D"ltr"><blockquote type=3D"cite">On Aug 14, 2020, at 12:2=
8, Suhas Daftuar via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.li=
nuxfoundation.org" target=3D"_blank">bitcoin-dev@lists.linuxfoundation.org<=
/a>&gt; wrote:<br><br></blockquote></div><blockquote type=3D"cite"><div dir=
=3D"ltr">=EF=BB=BF<div dir=3D"ltr">Hi,<div><br></div><div>Back in February =
I posted a proposal for WTXID-based transaction relay[1] (now known as BIP =
339), which included a proposal for feature negotiation to take place prior=
 to the VERACK message being received by each side.=C2=A0 In my email to th=
is list, I had asked for feedback as to whether that proposal was problemat=
ic, and didn&#39;t receive any responses.</div><div><br></div><div>Since th=
en, the implementation=C2=A0of BIP 339 has been merged into Bitcoin Core, t=
hough it has not yet been released.</div><div><br></div><div>In thinking ab=
out the mechanism used there, I thought it would=C2=A0be helpful to codify =
in a BIP the idea that Bitcoin network clients should ignore unknown messag=
es received before a VERACK.=C2=A0 A draft of my proposal is available here=
[2].</div><div><br></div><div>I presume that software upgrading past protoc=
ol version 70016 was already planning to either implement BIP 339, or ignor=
e the wtxidrelay message proposed in BIP 339 (if not, then this would creat=
e network split concerns in the future -- so I hope that someone would spea=
k up if this were a problem).=C2=A0 When we propose future protocol upgrade=
s that would benefit from feature negotiation at the time of connection, I =
think it would be nice to be able to use the same method as proposed in BIP=
 339, without even needing to bump the protocol version.=C2=A0 So having an=
 understanding that this is the standard of how other network clients opera=
te would be helpful.</div><div><br></div><div>If, on the other hand, this i=
s problematic for some reason, I look forward to hearing that as well, so t=
hat we can be careful about how we deploy future p2p changes to avoid disru=
ption.</div><div><br></div><div>Thanks,</div><div>Suhas Daftuar</div><div><=
br></div><div>[1]=C2=A0<a href=3D"https://lists.linuxfoundation.org/piperma=
il/bitcoin-dev/2020-February/017648.html" target=3D"_blank">https://lists.l=
inuxfoundation.org/pipermail/bitcoin-dev/2020-February/017648.html</a><br><=
/div><div><br></div><div>[2]=C2=A0<a href=3D"https://github.com/sdaftuar/bi=
ps/blob/2020-08-generalized-feature-negotiation/bip-p2p-feature-negotiation=
.mediawiki" target=3D"_blank">https://github.com/sdaftuar/bips/blob/2020-08=
-generalized-feature-negotiation/bip-p2p-feature-negotiation.mediawiki</a><=
/div></div>
<span>_______________________________________________</span><br><span>bitco=
in-dev mailing list</span><br><span><a href=3D"mailto:bitcoin-dev@lists.lin=
uxfoundation.org" target=3D"_blank">bitcoin-dev@lists.linuxfoundation.org</=
a></span><br><span><a href=3D"https://lists.linuxfoundation.org/mailman/lis=
tinfo/bitcoin-dev" target=3D"_blank">https://lists.linuxfoundation.org/mail=
man/listinfo/bitcoin-dev</a></span><br></div></blockquote></div></blockquot=
e></div>

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