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SubjectRe: PPP feature request (Tx queue len + close)
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On Tue, Mar 05, 2002 at 02:05:00PM +1100, Paul Mackerras wrote:
> Jean Tourrilhes writes:
>
> > Problem : IrDA does its buffering (IrTTP is a sliding window
> > protocol). PPP does its buffering (1 packet in ppp_generic +
> > dev->tx_queue_len = 3). End result : a large number of packets queued
> > for transmissions, which result in some network performance issues.
>
> How much buffering does IrTTP do? How large is its window? It is
> much more critical IMO to reduce the buffering below ppp_generic than
> it is to reduce the buffering above it. The ppp_generic layer itself
> does as little buffering as possible.

IrTTP is another problem. If I were to use TCP instead of
IrTTP, would you still ask me to reduce the window size of TCP ? Let's
try to be fair...
I'm taking the approach that every little thing helps. There
is a trivial win in PPP, and I would be stupid to not exploit it.
On the other hand, you are right that with IrTTP. I was
spending the day investigating this issue. As usual with Linux-IrDA,
it's very messy. I think I will need some architecture change to
implement proper flow control between IrLAP and IrTTP. And then
qualify that with all IrTTP users :-(

> > Solution : could we allow the PPP channel to overwrite
> > dev->tx_queue_len ?
> > This is similar to the channel beeing able to set the MTUs and
> > other parameters...
>
> Not really, the channel can't set the bundle MTU, only its own MTU.
> It can set the header length (the desired amount of headroom) but that
> is really only an optimization.
>
> What would happen in the case where two channels connected to the
> same ppp unit want to set the queue length to two different values?

No idea, never had this case ;-) This is exactly for this
reason I ask you.

> In general I think it would be better to get pppd to set the transmit
> queue length than to have the channel magically influencing stuff two
> levels above it.

I must have missed this option. I'll look again in the pppd
man page. That may be good enough...
For stuff influencing level above, just think of
ap->chan.hdrlen. In my case, it goes from IrLAP to TCP via IrLMP,
IrTTP, IrNET, PPP and IP.

> Could you produce some numbers showing better throughput, fewer
> retransmissions, or whatever, with a smaller transmit queue length?

Don't have number, but I don't need number to know that.

> Paul.

Jean
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