Quote from: r.willis on April 15, 2013, 09:41:02 PM

I have trouble implementing parallel blockchain download (headers only). I read wiki back and forth, but do not understand:
1) What exactly is block locator object.
2) How to download headers before certain block (towards genesis block)
Now I can start from genesis and work up blockchain (using 0 as hash_stop). Is it possible to move in opposite direction (from newest blocks to genesis)?

Block locator is a list of block hashes that you know about, starting from your head, back towards the genesis block. The idea is that the remote peer can know which blocks/headers to send you. If it determines that your most recent blocks are actually now on a side chain, then it will work backwards through your block locator to find a block which is on the main chain and then send you blocks from there onwards. If it cannot find any blocks in your locator which are on the main chain, it will send you blocks from the genesis block.
Current implementations create a block locator roughly as follows,

Code:

var i = current head height
var stepSize = 1

while(i > 0) {
   insert the hash of the block at height i
   i = i - stepSize
   stepSize = stepSize * 2
}

So many more recent blocks are added, and as you work back, the steps get bigger (because of the decreased likelihood of deeper and deeper reorgs).
When downloading the headers before the last checkpoint, you can simply use the hash of the most recent header you have as the locator, i.e. only one hash.

EDIT:
As for working backwards, this is not possible. What is possible however is to hardcode checkpoint hashes, this way you can download the headers between each checkpoint separately enabling your parallel download. I have code to do exactly this, but I haven't bothered to test it to see if the speed improvement was worth it compared to downloading from a single peer. Others may see this as a waste of network resources perhaps.