rich1.0npub1lwrwp8dzn9x5nqc7d735e0nuwxh0nyz5ezkfpjzr332efdxw9asq4z8zjmhttps://nostr.ae/npub1lwrwp8dzn9x5nqc7d735e0nuwxh0nyz5ezkfpjzr332efdxw9asq4z8zjmnjumphttps://nostr.ae📅 Original date posted:2014-07-17 📝 Original message:OVERVIEW To improve block propagation, add a new block message that doesn't include transactions the peer is known to have. The message must never require an additional round trip due to any transactions the peer doesn't have, but should be compatible with peers sometimes forgetting transactions they have known. APPROACH For peers advertising support for squashed blocks: a node tracks what txes it knows each peer has seen (inv received, tx sent, tx appeared in competing block known to peer). Nodes push block contents as txes-not-already-known + txids-known. A node should be able to forget invs it has seen without invalidating what peers know about its known txes. To allow for this, a node assembles a bloom filter of a set of txes it is going to forget, and sends it to peers. The node can erase the txes as soon as no blocks requested before the filter was pushed are in flight (relying on the assumption that messages can be expected to be processed in order). When a node receives a forgotten-filter, it ORs it into its forgotten-filter for that peer. Any transactions matching the forgotten-filter are always included in full with a block. If the filter is getting full, the node can just clear it along with peer.setTxKnown. COSTS Bloom filtering: Since the bloom filter is likely to grow slowly and can be dropped when it is becoming full, a cheap set of hash functions and element size can be used to keep overhead more restricted than the bloom filtering done for spv. It's important for testing txes against the filter to be fast so that it doesn't delay pushing the block more than the squashing helps. Nodes currently forget txes rarely, so the bloom filters would only need to be used at all under conditions that are not currently common -- but I think they're important to include to allow for different node behavior in this regard in the future. Tracking txes known to peers: A multimap of txid->peerId would obviate the current setCurrentlyKnown, and would not take much more space since each additional peer adds about 1 peerId per txid (setCurrentlyKnown keeps a uint256 per peer per txid, although it tracks somewhat fewer txid per node). Potential vulnerabilities: - Since the bloom filters will have lower maximum overhead than the current SPV filters and can be dropped at will, this shouldn't enable any resource exhaustion attacks that aren't already possible. - A squashed block with bogus or missing data would be easily detected not to produce the correct merkle root for its BlockHeader. BENEFITS Assuming a fairly typical 500 tx block with transaction sizes averaging 300b (both on the low side), for a 150kb block: % pruned | block size reduction | relative size reduction -------- | -------------------- | ----------------------- 100 | 134 kB | 89% 50 | 67 kB | 45% 25 | 33.5 kB | 17% I've been doing some logging, and when my node pushes a block to a peer it seems to typically know that a peer has seen most of the txes in the block. Even in the case of a small block with only 25% known-known transactions, total network bandwidth saved is greater than the bloom filters transmitted unless a node is forgetting transactions so rapidly that it pushes new maximum-size forget-filters every block. So this is a net gain even in total bandwidth usage, but most importantly it's an improvement in block propagation rate and in how block propagation rate scales with additional transactions. IMPLEMENTATION QUESTIONS How should block squashing capability be advertised -- new service bit? Bloom filters: - How fast to test against could a suitable bloom filter be made? - How much memory would each filter need to take, at maximum? - Can the inputs all being 32 byte hashes be used to optimize filter hash calculations? ROADMAP If there's support for this proposal, I can begin working on the specific implementation details, such as the bloom filters, message format, and capability advertisment, and draft a BIP once I have a concrete proposal for what those would look like and a corresponding precise cost/benefit analysis. --kaz -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20140717/810d2f69/attachment.html>