Subject: Re: PPP problems
To: None <>
From: Mike Long <>
List: port-i386
Date: 11/18/1996 00:17:30
>From: (Nathaniel D. Daw)
>Date: 17 Nov 1996 16:54:40 -0500

>I had this precise problem when I started using PPP with
>NetBSD-current around May. Data is being dropped out of large packets
>when leaving your system, which is why telnet works fine (except
>things like pasting from the X clipboard), but bulk file and mail
>transfers off the machine fail. To see if this is what is happening,
>try ping -s with various sizes up to the interface MTU. I found that
>packet loss rates increased with the size of the packet until almost
>none got through for large packets.

I have also seen exactly the same problem.  It's existed for a while;
I posted some messages about it to one of the lists (I forget which)
about a year ago.

>I don't believe this problem is related to the well known serial
>interrupts problem in NetBSD, which the patches already mentioned
>address -- that causes data dropped on packets coming *in* to your
>machine, as I understand it. I think the problem was actually a flow
>control problem on my ISP's side; I switched ISPs and my problems went
>away, while different computer/OS/modem combinations exhibited the
>same symptoms. So when reviewing your configuration I would suggest
>you see if flow control is working locally and remotely.

A flow control problem is likely; I saw the problem with both PPP and
TTY sessions (using C-Kermit).  You can't exclude interrupts, because
IIRC the com driver is notified of line status changes (i.e., changes
in the state of CTS) via an interrupt.

>Another suggestion I have heard for similar problems is to increase
>the size of certain buffers in the tty subsystem; I think this perhaps
>alleviated a problem with NetBSD sometimes losing data when modem flow
>control was asserted and the tty code not informed quickly
>enough. This was discussed on this mailing list some months ago, but I
>no longer have the exact patch. Anyone remember this?

I got a patch from Phil Knaack (sp?), which I somewhat upgraded to
work with more recent -current.  I'm not using it anymore, because I
got a new modem which no longer exhibits the problem.  My new modem
uses a 16550AF instead of my old modem's (ASIC copy of a) 16450; it's
possible that this bug exists only for FIFOless UARTs.

If Phil doesn't speak up I'll post what I have, but I can't guarantee
that it still works.
Mike Long <>     <URL:>
VLSI Design Engineer         finger for PGP public key
Analog Devices, CPD Division          CCBF225E7D3F7ECB2C8F7ABB15D9BE7B
Norwood, MA 02062 USA       (eq (opinion 'ADI) (opinion 'mike)) -> nil