Quote from: knightdk on March 16, 2016, 08:04:51 PM

Quote from: jl777 on March 16, 2016, 07:28:09 PM

I apologize for my technical ineptitudes.

I am trying to understand how segwit saves blockchain space as that is what it is being marketed as and with a softfork in the upcoming weeks.

It saves space in that it does not require that the signatures be downloaded. Since clients that are syncing do not need verify the signatures of old transactions, the witness data is then not needed. This would mean that they do not need to download the witnesses and can thus save bandwidth and space. This is only for several years in the future after segwit has been used for a while.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

So i look at the BIP, I find each segwit tx needs a commitment hash, plus more. this is 32 bytes per tx that to my ignorant simple minded thinking is in addition to what would be needed if it was a normal tx.

Not every transaction needs that hash, just like how not every transaction now has its txid stored in the blockchain. Just like it is done today, the merkle root of the wtxids is stored, and that is it.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

Now I am being told that we dont need to count the space used by the witness data. This confuses me. I like to count all permanently required data as the permanent space cost.

The witnesses are not permanent and can be deleted. After a few years, they won't even need to be downloaded.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

I apologize for asking questions like this, but I am just a simple C programmer trying to allocate space for the segwit and noticing it seems to take more space for every tx. I await to be properly learned about how the commitment hash is not actually needed to be stored anywhere and how that would still allow a full node to properly validate all the transactions.

Checking the transaction hash is not part of the validation of transactions. Rather the signature is validated with the witness data and checks that the signature is valid for that transaction. The block contains a merkle root. This merkle root is calculated by hashing all of the transaction hashes. The same is done for the witness root hash which is just the hash of all of the wtxids.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

Or did I misunderstand? Does segwit mean that it is no business of full nodes to verify all the transactions?

See above.

Quote from: jl777 on March 16, 2016, 07:33:29 PM

OK, I apologize again for misunderstanding the BIPs
that is why I am posting questions.

It is okay to ask questions, but when you spread around false information (like the title of this thread) then it is not okay. I admit, sometimes I do say false things, but I usually do say things with stuff like "I think" or "from what I understand" to indicate that what I say may not be the truth.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

can you show me a specific rawtxbytes for a simple tx as it is now and what the required bytes are if it was a segwit? and for the segwit case it would need to be two sets of bytes, I think I understand that part well enough.

Kind of, I can't build one right now and I don't have a segnet node set up yet so I can't pull one from there. I will give you a similar example, pulled from the BIP. In this example there are two inputs, one from p2pk (currently used) and one from a p2wpkh (segwit but this format will probably not be used).

Take for example this transaction:

Code:

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

Here it is broken down:

Code:

nVersion: 01000000
marker: 00
flag: 01
txin: 02 fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f 00000000 494830450221008b9d1dc26ba6a9cb62127b02742fa9d754cd3bebf337f7a55d114c8e5cdd30be022040529b194ba3f9281a99f2b1c0a19c0489bc22ede944ccf4ecbab4cc618ef3ed01 eeffffff
ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a 01000000 00 ffffffff
txout: 02 202cb20600000000 1976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac
9093510d00000000 1976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac
witness 00
02 47304402203609e17b84f6a7d30c80bfa610b5b4542f32a8a0d5447a12fb1366d7f01cc44a0220573a954c4518331561406f90300e8f3358f51928d43c212a8caed02de67eebee01 21025476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357
nLockTime: 11000000

An upgraded node would request and receive this transaction like this. It would contain all of the witness data.

But a non-upgraded node (and a node that doesn't want witness data) would only receive

Code:

0100000002fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f00000000494830450221008b9d1dc26ba6a9cb62127b02742fa9d754cd3bebf337f7a55d114c8e5cdd30be022040529b194ba3f9281a99f2b1c0a19c0489bc22ede944ccf4ecbab4cc618ef3ed01eeffffffef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a0100000000ffffffff02202cb206000000001976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac9093510d000000001976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac11000000

which is considerably smaller than the witness serialization because it doesn't include the witness.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

If the combined overhead is as small as 3 bytes, per tx, then it is amazing. but I dont understand how it is done.

But still even if it is only 3 bytes more, it is more data required permanently, so it is reducing the total tx capacity per MB and it anti-scaling from that standpoint. I cannot speak to the scaling impact of optimizd signature handling, etc.

Those three bytes (and whatever other overhead because I am fairly sure there is other overhead in the witness data) are not part of the transaction that goes into the count of the size of a block.

Quote from: jl777 on March 16, 2016, 07:28:09 PM

Also, it seems a 2MB hardfork is much, much simpler and provides the same or more tx capacity. Why isnt that done before segwit?

Because Segwit adds more useful stuff and 2 Mb hard fork doesn't solve the O(n^2) hashing problem. The Classic 2 Mb hard fork had some weird hackish workaround for the O(n^2) hashing problem. In terms of lines of code, I believe that there was an analysis done on segwit and the classic hard fork that found that the amount of code lines added was roughly the same. This is because the hashing problem needed to be fixed with that hard fork.

I suppose you could also say that segwit is more "efficient". Segwit, with roughly the same amount of code as the classic hard fork, brings much more functionality to Bitcoin than a 2 Mb hard fork does.

my reaction was based on the answers I was getting and clearly it is a complex issue. segwit is arguably more changes to bitcoin than all prior BIP's combined. I dont think anybody would say otherwise.

Now please ignore the space savings for nodes that are not full nodes. I am assuming that to bootstrap a node it will need to get the witness data from somewhere, right? so it is needed permanently and thus part of the permanent HDD requirement.

I still dont fully understand how the size of the truncated tx+ witness data is as small as 2 bytes per tx + 1 byte per vin. But even if that is the case, my OP title is accurate. as N+2*numtx+numvins is more than N

that is the math I see.

Also I made the mistake of making sure the transaction hash matches for a transaction. I had assumed that if the transaction hash doesnt match, it is invalid rawbytes. Are you saying that we dont need to verify that the transaction hashes match? As you know verifying signatures is very time consuming compared to verifying txid. So if verifying txid is not available anymore, that would dramatically increase the CPU load for any validating node.

Before I go making new threads about that, let us wait for some clarity on this issue.

I think if the witness data is assumed to be there permanently, then we dont increase the CPU load 10x or more to have to validate sigs vs validate txid, so it would be a moot point.

but it negates your point that the witness data can just go away

James