Re: [Bitcoin-development] Squashing redundant tx data in blocks on the wire

[Gregory Maxwell <gmaxwell@gmail.com>]

On Thu, Jul 31, 2014 at 2:41 PM, Kaz Wesley <keziahw@gmail.com> wrote:
>> the need to have transmitted the transaction list [..] first
>
> 32 bits per transaction is at least double the communication overhead
> of the simple approach, and only offers a bound on the probability of
> needing a round trip.

"(e.g. from a reconciliation step first)" the list can be communicated
in the space roughly equal to the size of the difference in sets plus
coding the permutation from the permissible orderings.   If you did
have some "simple approach" that guaranteed that some transactions
would be present, then you could code those with indexes... the FEC
still lets you fill in the missing transactions without knowing in
advance all that will be missing.   (Also, the suggestion on the
network block coding page of using part of a cryptographic permutation
as the key means that for unknown transactions the transmission of the
new unknown keys is always goodput— doesn't add overhead)

It's "only a bound" but you can pick whatever bound you want,
including— if you send data equal to the missing amount, then it'll be
always successful, but no bandwidth savings.   Though if the transport
is unordered (e.g. UDP or non-blocking SCTP) even sending 100% of the
missing amount can save time by eliminating a round trip that might
otherwise be needed for a retransmission.