[bittorrent] Re: BitTorrent Digest, Vol 2, Issue 18

David P. Mott dpmott at sep.com
Mon May 2 10:41:14 EDT 2005



> From: Justin Cormack <justin at street-vision.com>
> Subject: Re: [bittorrent] BT protocol Q
>
> On Fri, 2005-04-29 at 20:27 +0200, Olaf van der Spek wrote:
>> On 4/29/05, Simon Spero <ses at unc.edu> wrote:
>>> On Apr 29, 2005, at 1:49 PM, Olaf van der Spek wrote:
>>>
>>>> A request is for a chunk (16/32 kb), not a piece.
>>>
>>> Yes, but there are  *really* nasty problems with requesting chunks
>>> that are not the same size as  pieces.
>>
>> Maybe you should tell *all* client developers about those nasty problems then.
>
> I think everyone knows this is the biggest problem with BT. As you cant
> verify the stuff you get until you get a piece the whole thing is really
> nasty.
>
> The spec specifically says that chunks cannot be the same size as pieces
> as well.
>
> j


It does?

I thought that it said this:
* You can request up to 2^17, and you typically request 2^15.
* Piece length is almost always a power of two, most commonly 2^18 = 256 K

So, I don't see any reason (except for very large meta files) why you 
couldn't make the piece length 2^17 or smaller, such that you could 
request entire pieces at once.

The large size of the meta files, however, is the other nasty part of BT, 
and has historically hindered scalability across large trackers because 
of the bandwidth required to download them.

(Somebody correct me if I'm wrong on any of that...)

All of that led to the extended discussion and foray into Merkle trees, 
which was supposed to keep the meta file small and still let you request 
entire (and small) pieces.

Still waiting for BT 2.0... any word on if/when it'll ever see the light 
of day?

-dpmott



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