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In-Reply-To: <201107071049.48131.andyparkins@gmail.com>
References: <201107071049.48131.andyparkins@gmail.com>
Date: Thu, 7 Jul 2011 17:42:12 +0200
Message-ID: <CANEZrP0L-8PmwLma4DJdfoj+NefXS0kH8wvVFe-vuyRnpF-+mw@mail.gmail.com>
From: Mike Hearn <mike@plan99.net>
To: Andy Parkins <andyparkins@gmail.com>
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Suggestion for enhancements to getblock
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On Thu, Jul 7, 2011 at 11:49 AM, Andy Parkins <andyparkins@gmail.com> wrote=
:
> Imagine this situation though. =C2=A0I am a light weight client. =C2=A0I =
store the block
> headers only. =C2=A0I am only interested in the history of my own wallet =
addresses.
> I receive a block broadcast with a transaction that sends coins to one of=
 my
> addresses. =C2=A0That transaction references other transactions (of cours=
e), but I
> haven't stored any transactions. =C2=A0So; I want to request those transa=
ctions and
> ensure they are all valid and in blocks. =C2=A0I can't.

Everyone writing an alternative client goes through this thought
process :-) There's no point in doing it, you cannot prove your
transaction is not a double spend. That requires knowledge (ie, an
index) of all transactions.

You have to treat appearing deep in the chain as ipso-facto proof of
validity. Lightweight/SPV clients simply must have that trust, it
cannot be done any other way. See this article:

http://code.google.com/p/bitcoinj/wiki/SecurityModel

Currently this is pretty safe due to the crazy speeds. In future when
speeds are likely to be lower, it will be less safe and you'd have to
wait longer or use a trusted node.

> It should be possible to request the current pending transaction list.

I think it'd be better to implement the filtering suggestions that
have been made. It doesn't scale to download the entire memory pool -
a better approach is to give the remote node a filter to match against
transactions then have it only relay those. After setting a filter,
transactions pending and matching would be sent in one big inv and you
can then keep the connection open to learn about new transactions
without needing to "drink from the firehose". Filters can be
probabilistic and set on many different nodes to reduce the privacy
implications.