[Bitcoin-development] Suggestion for enhancements to getblock

[Andy Parkins <andyparkins@gmail.com>]

Hello,

This is a suggestion with a mind to the future.  In particular, I'm slowly 
working on an alternative client and library (I know, everyone says that).  
I've got a feature that I'd like to have that would need a change in the 
protocol.  It's a change that I think would improve the official client as 
well, so I'm bringing it up here.

It's actually two changes; and both could be acheived by adding new commands 
to the protocol.  I think that would be overkill though; as they fit quite 
nicely into an extended getblocks command.

(1) The getblocks message is a list of inventory items (type MSG_BLOCK) being 
requested.

Imagine this situation though.  I am a light weight client.  I store the block 
headers only.  I 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.  That transaction references other transactions (of course), but I 
haven't stored any transactions.  So; I want to request those transactions and 
ensure they are all valid and in blocks.  I can't.

I can request the transactions themselves; but I have no way of finding out 
what block they were in without downloading the entire full block chain 
myself.  The thing is, a peer with the full block chain is able to do this 
instantly.

Here then is my suggestion:  getblocks should accept inventory items of type 
MSG_TX as well as MSG_BLOCK.  When it finds a MSG_TX request, it shouldn't 
send the transaction (after all that is what the getdata message is for); 
instead it should return the block that contains that transaction.  It's an 
alternative way of requesting a block -- by transaction in that block.

It should be obvious then that it would be easy for a lightweight client to 
request the transaction chain transactions its interested in to create a list 
of relevant hashes for the history it's after; and then put those hashes in a 
getblocks request and have to look only at a few full blocks instead of the 
full block chain.


(2) If you are offline when new transactions are broadcast, there is no way to 
know they are pending.

Transactions that have been broadcast but not yet accepted into a block are 
never resent (nor should they be).  But if I am on a mobile client say, or a 
light-weight, intermittently used client; I have no way of checking if a 
transaction sending coins to one of my addresses is pending.

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

My sugestion then is that a special virtual block request be possible.  The 
all zero block hash can never exist (it's used as the parent for the genesis 
block, so it had better not).  If I send a getblocks that contains an all-zero 
hash, then the virtual block should be returned in a block message.  That 
virtual block will list all the current pending transactions.  Obviously there 
would be a problem that it's actual hash would not be zero; so it would have 
to be marked as the virtual block in some other way (perhaps a zero timestamp 
plus a zero parent hash, or similar).



The combination of the above two protocol changes makes it possible for any 
client to have a large chunk of the facilities available in blockexplorer.com 
and the very useful http://bitcoincharts.com/bitcoin/ information.




Andy
-- 
Dr Andy Parkins
andyparkins@gmail.com