[Bitcoin-development] Missing fRelayTxes in version

Mike Hearn mike at plan99.net
Thu Jun 20 10:58:31 UTC 2013


As I said, there's no benefit. Even if we do that on the C++ side, you
still have to handle connections from bitcoinj clients which will send the
field with the old version number. You can't assume they'll all be updated
simultaneously, even though both the Android app and MultiBit do have
update notifications these days and eventually old versions will presumably
disappear.

Re: flexibility. Let's say version V+1 adds a complicated new set of data
to some messages. Not every client wants or needs the feature enabled by
them.

Now version V+2 adds a simple extension to a basic message that everyone
wants/needs.

To get the latter feature, all clients now have to support the first
feature as well because the version number is monotonic.

OK, we can use a service bit to handle these cases, if we anticipate that
not all clients will want the first feature. But then again, we can also
use the presence of the additional data as the ground truth instead of
duplicating that fact. I don't really mind either way. It just seems that
parsing always requires you to be able to handle truncated messages anyway
(without asserting or crashing), because a bogus client can always send you
partial data. So I don't see what effort is saved.



On Thu, Jun 20, 2013 at 12:52 PM, Pieter Wuille <pieter.wuille at gmail.com>wrote:

> Let's just increase the version number and be done with this discussion.
> It's a small benefit, but it simplifies things and it's trivial to do.
>
> I don't understand how a policy of requiring version increases could limit
> future extensions: after the version/verack exchange, the protocol version
> is negotiated between peers, and there is no need for anything optional
> anymore.
>
> Note thay this is just about parsing, not about relaying - you should
> still relay parts of a message you haven't parsed. But that doesn't apply
> to the version message anyway, which is the only place where this matters.
>
> --
> Pieter
>  On 20 Jun 2013 12:38, "Turkey Breast" <turkeybreast at yahoo.com> wrote:
>
>> I don't get why this is such a contentious change?
>>
>> Before I was able to use asserts to check the expected length of length
>> of messages per protocol version, I could pass in dumb iterators that just
>> parse the byte stream and I could serialize and deserialize a message to
>> check the parser is correct (in debug mode).
>>
>> This 'simple' change causes all that behaviour to be lost. You can no
>> longer just use iterators but must know the remaining length (or if you use
>> std::distance, you can only use specific std containers - not just anything
>> with an iterator and an operator++). You cannot check the deserialization
>> process by serializing the deserialized message and comparing it to the
>> original data (because the bool is always present in the serializer).
>>
>> It's a bit stupid you call it buggy code when this behaviour has never
>> been present in Bitcoin. The BIP doesn't introduce any unwanted
>> side-effects and is a trivial reasonable change.
>>
>> If you want optional fields then the proper way to do it, is to either
>> set a flag in the Services field of the "version" message to indicate
>> different formats for messages (i.e use this template structure for a
>> message, not that one), introduce a new message (if the changes are big),
>> to approve/improve Stefan's BIP 32 for custom services or to have a value
>> in the byte stream indicating which fields are present (maybe a bitfield or
>> so).
>>
>> Using a quirk of an implementation is just bad form and sloppy coding.
>> Optional fields should have their own mechanism that allows them to remain
>> as optional fields between protocol version upgrades.
>>
>> The bitcoind software can probably be improved too, by checking that the
>> length of the version message is consistent for the protocol version given
>> by the connected node. Right now it makes no assumptions based on that
>> which is a mistake (new clients can broadcast older version messages that
>> don't have all the fields required). Probably the software should penalise
>> hosts which do that.
>>
>> What's the big deal to update the protocol version number from 70001 to
>> 70002? It's not like we'll run out of integers. The field has now gone from
>> optional to required now anyway - that's a behaviour change. It'd be good
>> to enforce that. I see this as a bug.
>>
>>   ------------------------------
>>  *From:* Mike Hearn <mike at plan99.net>
>> *To:* Pieter Wuille <pieter.wuille at gmail.com>
>> *Cc:* Bitcoin Dev <bitcoin-development at lists.sourceforge.net>; Tamas
>> Blummer <tamas at bitsofproof.com>
>> *Sent:* Thursday, June 20, 2013 11:17 AM
>> *Subject:* Re: [Bitcoin-development] Missing fRelayTxes in version
>>
>> 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 at 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
>>
>>
>>
>>
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