rich1.0npub1zw7cc8z78v6s3grujfvcv3ckpvg6kr0w7nz9yzvwyglyg0qu5sjsqhkhpxhttps://nostr.ae/npub1zw7cc8z78v6s3grujfvcv3ckpvg6kr0w7nz9yzvwyglyg0qu5sjsqhkhpxnjumphttps://nostr.ae📅 Original date posted:2016-05-10 📝 Original message:Gregory Maxwell <greg at xiph.org> writes: > On Tue, May 10, 2016 at 5:28 AM, Rusty Russell via bitcoin-dev > <bitcoin-dev at lists.linuxfoundation.org> wrote: >> I used variable-length bit encodings, and used the shortest encoding >> which is unique to you (including mempool). It's a little more work, >> but for an average node transmitting a block with 1300 txs and another >> ~3000 in the mempool, you expect about 12 bits per transaction. IOW, >> about 1/5 of your current size. Critically, we might be able to fit in >> two or three TCP packets. > > Hm. 12 bits sounds very small even giving those figures. Why failure > rate were you targeting? That's a good question; I was assuming a best-case in which we have mempool set reconciliation (handwave) thus know they are close. But there's also an alterior motive: any later more sophisticated approach will want variable-length IDs, and I'd like Matt to do the work :) In particular, you can significantly narrow the possibilities for a block by sending the min-fee-per-kb and a list of "txs in my mempool which didn't get in" and "txs which did despite not making the fee-per-kb". Those turn out to be tiny, and often make set reconciliation trivial. That's best done with variable-length IDs. > (*Not interesting because it mostly reduces exposure to loss and the > gods of TCP, but since those are the long poles in the latency tent, > it's best to escape them entirely, see Matt's udp_wip branch.) I'm not convinced on UDP; it always looks impressive, but then ends up reimplementing TCP in practice. We should be well within a TCP window for these, so it's hard to see where we'd win. >> I would also avoid the nonce to save recalculating for each node, and >> instead define an id as: > > Doing this would greatly increase the cost of a collision though, as > it would happen in many places in the network at once over the on the > network at once, rather than just happening on a single link, thus > hardly impacting overall propagation. "Greatly increase"? I don't see that. Let's assume an attacker grinds out 10,000 txs with 128 bits of the same TXID, and gets them all in a block. They then win the lottery and get a collision. Now we have to transmit ~48 bytes more than expected. > Using the same nonce means you also would not get a recovery gain from > jointly decoding using compact blocks sent from multiple peers (which > you'll have anyways in high bandwidth mode). Not quite true, since if their mempools differ they'll use different encoding lengths, but yes, you'll get less of this. > With a nonce a sender does have the option of reusing what they got-- > but the actual encoding cost is negligible, for a 2500 transaction > block its 27 microseconds (once per block, shared across all peers) > using Pieter's suggestion of siphash 1-3 instead of the cheaper > construct in the current draft. > > Of course, if you're going to check your whole mempool to reroll the > nonce, thats another matter-- but that seems wasteful compared to just > using a table driven size with a known negligible failure rate. I'm not worried about the sender: The recipient needs to encode all the mempool. >> As Peter R points out, we could later enhance receiver to brute force >> collisions (you could speed that by sending a XOR of all the txids, but >> really if there are more than a few collisions, give up). > > The band between "no collisions" and "infeasible many" is fairly > narrow. You can add a small amount more space to the ids and > immediately be in the no collision zone. Indeed, I would be adding extra bits in the sender and not implementing brute force in the receiver. But I welcome someone else to do so. Cheers, Rusty.