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Date: Sun, 28 Apr 2013 23:10:54 -0700
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Service bits for pruned nodes
Precedence: list

On 4/28/2013 8:55 PM, Peter Todd wrote:
> On Mon, Apr 29, 2013 at 03:48:18AM +0000, John Dillon wrote:
>> We can build this stuff incrementally I'll agree. It won't be the case that one
>> in a thousand nodes serve up the part of the chain you need overnight. So many
>> I am over engineering the solution with BitTorrent.
>
> I think that pretty much sums it up.
>
> With the block-range served in the anounce message you just need to find
> an annoucement with the right range, and at worst connect to a few more
> node to get what you need.

One of the technologies that can be borrowed from Bittorrent (besides 
downloading from multiple peers at once) is analysis by clients of the 
part distribution, which allows a client to download and share the 
least-propagated parts first to maintain high availability of the whole 
file, even when not one individual currently has downloaded the complete 
file (the seed has left the swarm).

Unlike Bittorrent, a partial-blockchain swarm client needs to make 
informed decisions about how much to download, such as rules like "until 
it sees at least 20 complete blockchain-equivalents in the swarm", 
"until it has 10% of the blockchain itself", "work backwards, all blocks 
from the hash tree required to verify my payments" or other minimums 
that might all be criteria.

Bittorrent only considers directly connected peers' piecemaps when 
making decisions of what to download. Bitcoin, however, already has a 
protocol to allow peer discovery beyond the connected nodes; this could 
be extended to communicate what parts the peer is hosting. Careful 
thought into attack vectors would need to be paid in design, so that 
only a majority of outbound-connected peers's advertisement are able to 
inform consensus about part or peer availability, messages able to 
remove a peer or part availability from other's lists are confirmed 
independently without such removal verification generating DDOS traffic 
amplification, lying clients can be marked as discovered by the 
majority, etc.

Such thought doesn't have to be paid if directly implementing 
Bittorrent, but it has the burden of centralized trackers or expensive 
DHT, and it also doesn't have any logic informing it besides "don't quit 
until I get the whole file".