There's no problem, but there's no benefit either. It also locks us in to a
potentially problematic guarantee - what if in future we want to have, say,
two optional new pieces of data in two different messages. We don't want to
require that if version > X then you have to implement all features up to
and including that point.

Essentially the number of fields in a message is like a little version
number, just for that message. It adds flexibility to keep it that way, and
there's no downside, seeing as that bridge was already crossed and people
with parsers that can't handle it need to fix their code anyway.

So I have a slight preference for keeping things the way they are, it keeps
things flexible for future and costs nothing.



On Thu, Jun 20, 2013 at 11:06 AM, Pieter Wuille <pieter.wuille@gmail.com>wrote:

> On Thu, Jun 20, 2013 at 09:36:40AM +0200, Mike Hearn wrote:
> > Sure but why not do that when there's an actual new field to add? Does
> > anyone have a proposal for a feature that needs a new version field at
> the
> > moment? There's no point changing the protocol now unless there's
> actually
> > a new field to add.
> >
> > Anyway I still don't see why anyone cares about this issue. The Bitcoin
> > protocol does not and never has required that all messages have a fixed
> > number of fields per version. Any parser written on the assumption it did
> > was just buggy. Look at how tx messages are relayed for the most obvious
> > example of that pattern in action - it's actually the raw byte stream
> > that's stored and relayed to ensure that fields added in new versions
> > aren't dropped during round-tripping. Old versions are supposed to
> preserve
> > fields from the future.
>
> Actually, that is not the same issue. What is being argued for here is that
> the version in the version message itself should indicate which fields are
> present, so a parser doesn't need to look at the length of the message.
> That
> seems like a minor but very reasonable request to me, and it's trivial to
> do.
> That doesn't mean that you may receive versions higher than what you know
> of,
> and thus messages with fields you don't know about. That doesn't matter,
> you
> can just ignore them.
>
> I see no problem with raising the protocol version number to indicate
> "all fields up to fRelayTxes are required, if the announced nVersion is
> above N".
> In fact, I believe (though haven't checked) all previous additions to the
> version
> message were accompanied with a protocol version (then: client version)
> increase
> as well.
>
> --
> Pieter
>
>