[bitcoin-dev] Compact Block Relay BIP

[Rusty Russell]

Pieter Wuille via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> writes:
> On 05/03/2016 12:13 AM, lf-lists at mattcorallo.com (Matt Corallo) wrote:
>> Hi all,
>> 
>> The following is a BIP-formatted design spec for compact block relay
>> designed to limit on wire bytes during block relay. You can find the
>> latest version of this document at
>> https://github.com/TheBlueMatt/bips/blob/master/bip-TODO.mediawiki.
>
> Hi Matt,
>
> thank you for working on this!

Indeed!  Sorry for the delayed feedback.

>> |shortids||List of uint64_ts||8*shortids_length bytes||Little
>> Endian||The short transaction IDs calculated from the transactions which
>> were not provided explicitly in prefilledtxn
>
> I tried to derive what length of short ids is actually necessary (some
> write-up is on
> https://gist.github.com/sipa/b2eb2e486156b5509ac711edd16153ed but it's
> incomplete).

I did this for IBLT testing.

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.

The wire encoding of all those bit arrays was:
  [varint-min-numbits] - Shortest bit array length
  [varint-array-size]  - Number of bit arrays.
          [varint-num].... - Number of entries in array N (x varint-array-size)
  [packed-bit-arrays...]

  Last byte was padded with zeros.
  See: https://github.com/rustyrussell/bitcoin-iblt/blob/master/wire_encode.cpp#L12

I would also avoid the nonce to save recalculating for each node, and
instead define an id as:

        [<64-bit-short-id>][txid]

Since you only ever send as many bits as needed to distinguish, this only
makes a difference if there actually are collisions.

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).

And a prototype could just always send 64-bit ids to start.

Cheers,
Rusty.