[bitcoin-dev] [RFC] IBLT block testing implementation

[Kalle Rosenbaum]

2015-06-23 7:53 GMT+02:00 Rusty Russell <rusty at rustcorp.com.au>:
> Hi all,
>
>         I've come up with a model for using IBLT to communicate blocks
> between peers.  The gory details can be found on github: it's a
> standalone C++ app for testing not integrated with bitcoin.
>
>         https://github.com/rustyrussell/bitcoin-iblt

Good to see that you're working on this. Really exciting!

I want to take the opportunity to link to my previous work on IBLTs, for
those that haven't seen it, where I investigate the behaviour of the IBLT
when changing different parameters, like cell count, hashFunctionCount etc:
https://github.com/kallerosenbaum/bitcoin-iblt/wiki

>From glancing over your implementation, I see that you don't use a
keyHashSum, in fact you don't use a key at all, but only a
concatenatenation of (txid48, fragid, tx-chunk) as value. Here the
txid48+fragid functions as a kind of keyHashSum. I think this might be a
very good idea,

If you have a false positive with count == 1, then you would probably
detect it if fragid is outside reasonable limit from from base_fragid. Did
you implement your idea to remove all the count==1 fagments in ascending
order of (fragid-base_fragid)?

Anyhow, I think we should make some more comparable tests, just as you
proposed last year when I didn't reply, sorry... My code is a more straight
forward implementation of the IBLT paper (http://arxiv.org/pdf/1101.2245.pdf)
and encoding blocks is done pretty much as Gavin proposed in his gist. That
should function as a baseline so that we can validate that your
optimizations actually work. Please contact me directly if you are
interested in that, and we'll figure something out.

More comments/questions inline:

>
> Assumptions and details:
>
> 1. The base idea comes from Gavin's Block Propagation gist:
>         https://gist.github.com/gavinandresen/e20c3b5a1d4b97f79ac2
>
> 2. It relies on similarity in mempools, with some selection hints.  This
>    is designed to be as flexible as possible to make fewest assumptions
>    on tx selection policy.
>
> 3. The selection hints are: minimum fee-per-byte (fixed point) and
>    bitmaps of included-despite-that and rejected-despite-that.  The
>    former covers things like child-pays-for-parent and the priority
>    area.  The latter covers other cases like Eligius censoring "spam",
>    bitcoin version differences, etc.
>

There is a chance that the bit prefix of the added or removed tx is not
unique within the receiver's mempool. In that case the receiver can
probably just use the earliest matching transaction and hope for the best.
If not -> bad luck. Is that how you do it?

> 4. Cost to represent these exceptional added or excluded transactions is
>    (on average) log2(transactions in mempool) bits.

These exceptional tx *could* instead be encoded in the IBLT, just as if
they were unknown differences. Your bitmaps is probably a more compact
representation, but it's also more complex.

>
> 5. At Peiter Wuille's suggestion, it is designed to be reencoded between
>    nodes.  It seems fast enough for that, and neighboring nodes should
>    have most similar mempools.
>

What is the purpose of reencoding when relaying? Is that to improve the
failure probability as new tx may have arrived in the mempool of the
intermediary node?

> 6. It performs reasonably well on my 100 block sample in bitcoin-corpus
>    (chosen to include a mempool spike):
>
>    Average block range (bytes):                         7988-999829
>    Block size mean (bytes):                             401926
>
>    Minimal decodable BLT size range (bytes):            314-386361
>    Minimal decodable BLT size mean (bytes):             13265
>
> 7. Actual results will have to be worse than these minima, as peers will
>    have to oversize the IBLT to have reasonable chance of success.
>

Yes, I have made some analysis on this here:
https://github.com/kallerosenbaum/bitcoin-iblt/wiki/FailureProbability. We
use quite different data structures for encoding blocks in IBLT, but it
might apply to your implementation as well. An interesting result is that
the space saving percentage actually increases as blocks grow.

> 8. There is more work to do, and more investigation to be done, but I
>    don't expect more than a 25% reduction in this "ideal minimum"
>    result.
>
> Special thanks to Kalle Rosenbaum for helping investigate IBLTs with me
> last year.

Thank you too!

Regards,
Kalle

>
> Cheers,
> Rusty.
> PS. I work for Blockstream.  And I'm supposed to be working on
>     Lightning, not this.
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