Subject: Re: [bitcoin-dev] Compact Block Relay BIP

From: Pieter Wuille <[email-masked]>
Date: Mon, 9 May 2016 19:06:06 +0200

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!

> ===New data structures===
> Several new data structures are added to the P2P network to relay
> compact blocks: PrefilledTransaction, HeaderAndShortIDs,
> BlockTransactionsRequest, and BlockTransactions. Additionally, we
> introduce a new variable-length integer encoding for use in these data
> structures.
> 
> For the purposes of this section, CompactSize refers to the
> variable-length integer encoding used across the existing P2P protocol
> to encode array lengths, among other things, in 1, 3, 5 or 9 bytes.

This is a not, but I think it's a bit strange to have two separate
variable length integers in the same specification. I understand is one
is already the default for variable-length integers currently, and there
are reasons to use the other one for efficiency reasons in some places,
but perhaps we should aim to get everything using the latter?

> ====New VarInt====
> Variable-length integers: bytes are a MSB base-128 encoding of the number.
> The high bit in each byte signifies whether another digit follows. To make
> sure the encoding is one-to-one, one is subtracted from all but the last
> digit.

Maybe it's worth mentioning that it is based on ASN.1 BER's compressed
integer format (see
https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
section 8.1.3.5), though with a small modification to make every integer
have a single unique encoding.

> ====HeaderAndShortIDs====
> A HeaderAndShortIDs structure is used to relay a block header, the short
> transactions IDs used for matching already-available transactions, and a
> select few transactions which we expect a peer may be missing.
> 
> |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).

For any reasonable numbers I can come up with (in a very wide range),
the number of bits needed is very well approximated by:

  log2(#receiver_mempool_txn * #block_txn_not_in_receiver_mempool /
acceptable_per_block_failure_rate)

For example, with 20000 mempool transactions, 2500 transactions in a
block, 95% hitrate, and a chance of 1 in 10000 blocks to fail to
reconstruct, needed_bits = log2(20000 * 2500 * (1 - 0.95) / 0.0001) =
34.54, or 5 byte txids would suffice.

Note that 1 in 10000 failures may sound like a lot, but this is for each
individual connection, and since every transmission uses separately
salted identifiers, occasional failures should not affect global
propagation. Given that transmission failures due to timeouts, network
connectivity, ... already occur much more frequently than once every few
gigabytes (what 10000 blocks corresponds to), that's probably already
more than enough.

In short: I believe 5 or 6 byte txids should be enough, but perhaps it
makes sense to allow the sender to choose (so he can weigh trying
multiple nonces against increasing the short txid length).

> ====Short transaction IDs====
> Short transaction IDs are used to represent a transaction without
> sending a full 256-bit hash. They are calculated by:
> # single-SHA256 hashing the block header with the nonce appended (in
> little-endian)
> # XORing each 8-byte chunk of the double-SHA256 transaction hash with
> each corresponding 8-byte chunk of the hash from the previous step
> # Adding each of the XORed 8-byte chunks together (in little-endian)
> iteratively to find the short transaction ID

An alternative would be using SipHash-1-3 (a form of SipHash with
reduced iteration counts; the default is SipHash-2-4). SipHash was
designed as a Message Authentication Code, where the security
requirements are much stronger than in our case (in particular, we don't
care about observers being able to finding the key, as the key is just
public knowledge here). One of the designers of SipHash has commented
that SipHash-1-3 for collision resistance in hash tables may be enough:
https://github.com/rust-lang/rust/issues/29754#issuecomment-156073946

Using SipHash-1-3 on modern hardware would take ~32 CPU cycles per txid.

> ===Implementation Notes===

There are a few more heuristics that MAY be used to improve performance:

* Receivers should treat short txids in blocks that match multiple
mempool transactions as non-matches, and request the transactions. This
significantly reduces the failure to reconstruct.

* When constructing a compact block to send, the sender can verify it
against its own mempool to check for collisions, and if so, choose to
either try another nonce, or increase the short txid length.

Cheers,

-- 
Pieter