I think it is time to move forward with pruning nodes, i.e. nodes that fully validate and relay blocks and transactions, but which do not keep (all) historic blocks around, and thus cannot be queried for these.

The biggest roadblock is making sure new and old nodes that start up are able to find nodes to synchronize from. To help them find peers, I would like to propose adding two extra service bits to the P2P protocol:

* NODE_VALIDATE: relay and validate blocks and transactions, but is only guaranteed to answer getdata requests for (recently) relayed blocks and transactions, and mempool transactions.

* NODE_BLOCKS_2016: can be queried for the last 2016 blocks, but without guarantee for relaying/validating new blocks and transactions.

* NODE_NETWORK (which existed before) will imply NODE_VALIDATE and guarantee availability of all historic blocks.

The idea is to separate the different responsibilities of network nodes into separate bits, so they can - at some point - be implemented independently. Perhaps we want more than just one degree (2016 blocks), maybe also 144 or 210000, but those can be added later if necessary. I monitored the frequency of block depths requested from my public node, and got this frequency distribution: http://bitcoin.sipa.be/depth-small.png so it seems 2016 nicely matches the set of frequently-requested blocks (indicating that few nodes are offline for more than 2 weeks consecutively.

I'll write a BIP to formalize this, but wanted to get an idea of how much support there is for a change like this.

Cheers,

-- 

Pieter