XThin vs Compact Blocks

Slides from presentation given at:
"Satoshi’s Vision: Bitcoin Development & Scaling Conference" September 25, 2016

Westin St. Francis, San Francisco

Transcript

1. XThin vs Compact Blocks Technical comparison. How they are similar - and not.
2. Who am I? I’m Dagur. dagurval on github. I often hack on Bitcoin XT.
3. 2 minute crash course in Compact Blocks … by comparing protocol basics. Asumes you understand xthins.
4. Announce support XThin Service bit: NODE_XTHIN = (1 << 4), Compact Blocks Message: sendcmpct
5. Request block XThin Message: getdata Inv(MSG_XTHINBLOCK) (or MSG_THINBLOCK) Compact Blocks Message: getdata Inv(MSG_CMPCT_BLOCK)
6. That one was a bit too similar! MSG_THINBLOCK== 4 MSG_CMPCT_BLOCK == 4
7. That one was a bit too similar!
8. That one was a bit too similar!
9. That one was a bit too similar!
10. This identifier conflict is OK! It’s a bit more work to support both. We only have to ignore one of BIP152 (Compact Blocks) requirements.
11. Sending blocks XThin Message: xthinblock (or thinblock) Compact Blocks Message: cmpctblock
12. Requesting missing transactions XThin Message: get_xblocktx Compact Blocks Message: getblocktxn
13. Sending missing transactions XThin Message: xblocktx Compact Blocks Message: blocktxn
14. Protocol flow compared Protocols compared. (Modified diagram from BIP152)
15. So, it’s the same technology. It’s the differences that are interesting.
16. Announcing support is different • Using service bit is good for locating peers. • XThin supporting nodes are scarce. • Compact block would benifit less, plenty of Core nodes.
17. XThin sends a filter with a block request. Compact blocks does not • Filter represents the requesters mempool • Using a filter increases request by Average: ~5KB (BU) Protocol Max: 36KB. Prefill accuracy: XThin: ~1% re-request rate with 5KB filter. Compact Blocks: ~60% re-request rate with current algorithm. Big improvement potential.
18. XThin sends a filter with a block request. Compact blocks does not Mempool policies: XThin benifits from similar mempool policies Compact Blocks requires similar mempool policies My opinion: • Having a filter is worth the extra bytes • Filter is more cross-client friendly.
19. Encoding with hashes or indexes • XThin and Compact Blocks both send list of 64bit hashes in a block. • Compact Blocks obfuscates them. • Compact Blocks adds index to prefilled txs. ◦ Saves: 1KB per 21 prefilled
20. Encoding with hashes or indexes • Transactions have a fixed position in the block. • Compact Blocks use this in re-requests. Savings: ~1KB per 21 txs re-requested. My opinion: • Using index is a good idea. • Compact Blocks encoding is needlessly complex.
21. Scalability • XThin can scale as is. • Compact Blocks gets issues when blocks have >= 2^16 transactions. (~38MB blocks with avg 0.6KB per tx. ~17MB with carefully crafted blocks) • Easy to fix by updating index to 32bit integer or with a hack.
22. Upgrading the protocol • XThin does not have a defined upgrade mechanism • Compact Blocks define a version in their handshake. • Nodes ignore Compact Block support if versions don’t match theirs.
23. Segwit capability • Compact blocks bumps its version number. • Peers with version 2 get segwit serialized transactions. • XThin can also be made compatible with segwit by changing the serialization.
24. Low latency relaying Pushing blocks is back in style! • Compact Blocks has a “high bandwith mode” • Similar to xpedited - both save a round trip
25. Basic verification Send xthin block Block received Full verification Xpedited A Send Compact Block Block received Full verification Compact Blocks High bandwith mode A B B VS
26. Low latency relaying Pushing blocks is back in style! • Compact Blocks requests announcements with blocks from 3 semi-arbitrary peers. • Xpedited is configured in bitcoin.conf
27. How are hash collisions in mempool handled? • XThin falls back to requesting thicker blocks (256bit hashes) • Compact block explicitly re-requests the transactions that collided • XThin nodes can also use the re-request strategy (XT does)
28. My conclusion • There isn’t much difference. • Compact Blocks is more complex to implement. • Compact Block manages to shave of some more bytes. • I doubt there will be performance difference. I hope someone does a performance comparison. • Overall both are good protocols.
29. Thank you. Questions?