[BitTorrent] Re: bt2 protocol features

Olaf van der Spek OvdSpek at LIACS.NL
Fri Jun 11 16:48:36 EDT 2004


> >From: Olaf van der Spek <OvdSpek at LIACS.NL>
> > Elliott Mitchell wrote:
> > >>From: Olaf van der Spek <OvdSpek at LIACS.NL>
> > >>Elliott Mitchell wrote:
> > >>
> > >>>>From: Olaf van der Spek <OvdSpek at LIACS.NL>
> > >>>>Yes, a piece size of between 16 kb and 64 kb would be required and
that
> > >>>>would significantly increase the protocol overhead compared with 2
mb
> > >>>>pieces.
> > >>>
> > >>>
> > >>>Really?
> > >>
> > >>You'd have to send more HAVEs and you need more hashes.
> > >
> > > Yes, but how large is that? 8-12 additional bytes is not much
additional
> > > data unless you're sending out *lots* of them (either a tiny block
size
> > > or a massive number of peers).
> >
> > Or both.
> > What's your definition of a tiny block size?
> > And of a massive number of peers?
>
> Well, I've already pointed out that for 256K blocks (current default for
> the mainstream) and 50 peers (towards the upper end for the mainstream),
> the overhead is less than 0.5% of the size of the payload.
>
> So looking at the worst case. Is 100 peers and 16KB chunks a sufficiently
> ridiculous for you? At this point we get 1 REQUEST message, 1 PIECE

100 peers isn't that much, but it's a reasonable case.

> message, and 100 HAVE messages for each chunk. So an additional 1632

16 bytes per HAVE?
You'd need extra HAVE combining logic for that and even then you probably
need to take into account IP/TCP overhead.

> bytes of overhead will be used, for a little less than 10% additional
> overhead for a *really* bad case.
>
> Of these two numbers I think the first (0.5%) is the better estimate,

A chunk size of 256 kb doesn't sound realistic.

> since with 16KB chunks the BT protocol overhead is already huge (on the
> order of 25% overhead by itself, without this feature; both are dying
> because of all the HAVE messages). Is this sufficient to settle bandwidth
> being an issue?
>
> > > So I'll admit care will be needed, but care will be needed anyway. If
> > > some requirement makes it absolutely impossible, _then_ this will need
to
> > > be removed. Until that time do you object to this remaining an idea to
> > > _try_ to put in place?
> > >
> > No.
>
> Why not? This seems like a pretty simple and reasonable idea.

I agree (and that's why I didn't object).



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