[bittorrent] UDP

Olaf van der Spek olafvdspek at gmail.com
Fri Apr 1 07:37:07 EST 2005


On Apr 1, 2005 1:23 PM, Joseph Ashwood <ashwood at msn.com> wrote:
> ----- Original Message -----
> From: "Olaf van der Spek" <olafvdspek at gmail.com>
> Subject: Re: [bittorrent] UDP
> 
> > On Apr 1, 2005 1:22 AM, Joseph Ashwood <ashwood at msn.com> wrote:
> >> ----- Original Message -----
> >> From: "Olaf van der Spek" <olafvdspek at gmail.com>
> >> Subject: Re: [bittorrent] UDP
> >>
> >> > The ACKs are also used for congestion/rate control. How would you do
> >> > that without ACKs?
> >>
> >> Two ways: First ignore the problem. Having to make a few extra requests
> >> for
> >> packets that have been dropped won't pose too much of a burden on the
> >> network. Second by requesting few enough packets that your connection
> >> won't
> >> be swamped. The overall connection rate can be estimated based on how
> >> high
> >> the incoming rate can be before it caps. Between these it should be
> >> possible
> >
> > I don't think that's going to work.
> 
> They are highly imprecise, but should offer at least some control.
> 
> > Congestion avoidance and rate
> > control are far too complex for such a simple solution.
> > What about the outgoing rate?
> 
> Outgoing rate is (to me) not as much of a concern, the underlying operating
> system should perform the necessary pipelining anyway. I'm not aware of any
> operating system that won't send everything but I'm far from an expert of
> this.

If you send UDP packets too fast, your operating system or some router
will just drop them.

> You seem to be forgetting (as I did for quite some time) that there are
> hidden ACKs in the protocol. Every request (except the last) has a response.

But what about the normal ACK to the response?

> I admit this pushes a significant (i.e. major) amount of work from the OS
> into the application. If the ACK.responses don't arrive then there is
> congestion, if the responses are arriving the congestion is missing,
> oversimplified yes, but the fundamentals are there.
> 
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