Standards (was [BitTorrent] Back to Merkle Hash Trees...)

Olaf van der Spek OvdSpek at LIACS.NL
Tue Feb 8 05:31:07 EST 2005


Justin Cormack wrote:
>>
>>Justin Cormack wrote:
>>
>>>>>32 bits for the piece. I thought everyone had pretty much agreed that 1k/4k
>>>>
>>>>2^47 bytes doesn't look like a 'big' limitation.
>>>
>>>
>>>Its a bit too small, 128TB, as it is within range of the size of filesystems
>>>people have now, let alone a few years in the future.
>>
>>Let's just say I'd love to hit that limitation.
> 
> 
> Better safe than sorry.

The largest torrents I've seen were about 8 gb. That's a factor 16384 
smaller then the limit. I doubt the protocol will still be in use when 
this becomes an issue.
But using 64-bit indexes would be an easy solution.


>>>>>your xbt url doesnt support a multifile torrent.
>>>>
>>>>Why not?
>>>>It's the info_hash that's included, not the root hash of a single file.
>>>
>>>
>>>But the info_hash isnt sufficient information to verify a torrent. Of course
>>
>>Why not?
>>With the info_hash, you can verify the info key and with the info key 
>>you can verify the rest.
> 
> 
> yes but without the Merkle root hashes you dont know where the error was.

But those root hashes are in the info key.

>>>one logical thing would be to Merkle hash the individual file hashes into
>>>one hash...
>>>
>>>ok re see belows above.
>>>
>>>One of the points of Merkle hashes is that you dont really need the piece/chunk
>>>distinction.
>>>
>>>I can see what you are doing, keeping the distinction means fewer have messages
>>>(though you negate this somewhat as the chunk_have messages clearly have a
>>>useful purpose, even if only at some times).
>>
>>It's not just that. It's also a smaller bitfield message and smaller 
>>in-memory bitfields.
> 
> 
> The overhead is not that great, especially if you give an offset and length
> you have verified in the have message. You can store internally as this too.

That sounds a lot more complex then the flat bit vector in use now 
(internally).

>>>There are a lot of options available once one gets this flexibility, and it
>>>is probably best to scrap the piece/chunk distinction.
>>>
>>>Call the smallest verifiable unit SVU (say 4k).
>>>
>>>One option would be that if have messages are a range of SVUs (or SVU + length)
>>>and requests have lengths then you could have a standard algorithm to ramp
>>>up request sizes as downloads progress, for example, or make this rarity
>>>based. This would amortise the slightly large have and request messages for
>>>small requests with those for larger requests later.
>>
>>I thought about something like that, but someoone else suggested I kept 
>>the protocol as simple as possible. I wanted to include a 32-bit vector 
>>in the request, along with a piece index. That'd allow you to request 
>>between 0 and 32 chunks of one piece with one request. Another 
>>optimization would be to allow multiple requests per request message.
>>The same could be done for have and cancel messages.
>>
>>Another issue is random access IO. If a seek takes 10 ms, you can only 
>>do 100 seeks per second and with chunks of 4 kb, that'd mean a top of 
>>400 kb. If you don't have pieces and peers requests chunks completely at 
>>random, you can't do read-ahead.
> 
> 
> Sorry, was there an extra dont in that last sentence?
> 
> Most of the time it makes sense to do transfers in large amounts at a time
> it is just at the beginning you might want a lower value.

True.

With 4k chunks, do you keep the entire merkle tree in memory?


 
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