[BitTorrent] BT2 & hash trees

Mirco Romanato painlord2k at libero.it
Wed Aug 11 12:21:06 EDT 2004


Elliott Mitchell wrote:

>>From: Mirco Romanato <painlord2k at libero.it>
>>Elliott Mitchell wrote:
>>    
>>
>_I_'m assuming something? Seems like you're almost agreeing that it is a
>non-trivial exercise to find the same file on Shareaza...
>  
>
This because the .torrent file don't contain the standard hashes of the 
individual files (SHA-1, TTH, CMD4).
Put the hashes and the exercise is trivial.



>>As for your questions, I think no and no.
>>But the questions could be others:
>>1) Does BT follow common standards with other P2P networks could give 
>>better performance/experience to users?
>>    
>>
>Certainly possible. I'm dubious about the direct gain though. I do
>strongly suspect using THEX may cause great harm to BT2, in which case
>the answer would be a strong no.
>  
>
My best guest is that the chunks size for files can be fixed to 
something like 256 or 512 KB (because BT2 is about trasmitting big files).
I think a .torrent2 file could include all the file's hashes 
(SHA1/TTH/ed2k/SHATreehash), then the clients will ask for the hash tree 
of a file to another client, when will ask for the first chunk of the file.

This would reduce the need to trasfer all the data in the same time (for 
multiple files torrent) and the load for any single source.


>>2) Does BT follow a different standard help its users?
>>    
>>
>I think this question is almost redundant. Using another hash arrangement
>will make multi-protocol clients more complex and /possibly/ harm users.
>If another hash arrangement allows for faster download that will help
>users. I will have to guess the latter will be the bigger deal. I
>/suspect/ trying to stick to THEX /may/ cause great harm to BT2. Not
>really enough in this question to change anything.
>  
>
Annother way to put the question is:
the gain to be incompatible is more than the gain to be compatible.

And, please, if you have any real suspect explain it.
I think feeling is not a good way to gauge this type of problems.

>CMD4?
>  
>
Composed MD4
The eDonkey hash AKA ed2khash

>I cannot dispute the thrust of the above idea. Recording the THEX-style
>hash of the file(s) in the .torrent should not be a great expense in file
>size. My concern is that using THEX-style hashes in the BT2 _protocol_
>will be a very bad thing.
>  
>

>I'm going to remain skeptical of the value of including that data, but
>the .torrent format easily allows for extra data like that. If that is
>all you want, then fine let it be an optional but defined field in
>.torrent files.
>  
>
This would help much the filesharing application to bridge the the 
bittorrent downloads with the net shared resources.

>They're cheap on a fast system with a 56kbps modem. On a slowish system
>with a 1gbps link processor cycles are fairly dear.
>  
>
Could you do math about this problem?
1) If I download a torrent with my new 1.280 Kbps line, what are the 
system requisites so I'm able to run the DL at top speed?
2) If I upload a torrent with my new 1.280 Kbps line, what are the 
system requisites so I'm able to run the UL at top speed?

FYI, now Shareaza is able to compute four hashes (SHA1, TTH, CMD4, MD5)  
at around 200 MB/min full speed (for my AthlonXP1800+), before the last 
assembly optimizations, that near doubled this numbers with 
PentiumIV/AthlonXP.
And for what I noted, near all the DLs are for the standard build and 
for this optimization.

>It will certainly be trivial to make it not require running the data
>through the hash twice to detect whether it is internal node versus leaf.
>  
>
The problems probably is trivial if you don't include evil clients 
interesting in distrupting the torrent.
But, if you come with a way to save CPU time that is secure against 
attacks, I think there is nothing bad to do this (as you let 
compatibility with the other standards).

>>The only real requirement is that all network and protocols will be 
>>implemented in a way that don't harm the other  clients and networks.
>>    
>>
>Well, you've noted one way that qualifies, recording alternative hashes
>in the .torrent files. This doesn't harm BT, while hopefully providing
>you all the help you need.
>  
>
Exactly.

Mirco



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