[MLB-WIRELESS] linux / atmel usb / fragmentation

[Donovan Baarda]

G'day,

On Mon, 2003-02-10 at 08:24, Matt wrote:
> Hi,
> I'm running a usb wireless connection and have had moderately common 
> intermittent connection issues. I've tried tweaking a number of things 
> (thinking I've had routing issues, perhaps even a weird routing loop) but 
> they haven't really taken care of the main problem. My device still 
> associates with the AP but I can't get packets through. Running tcpdump 
> shows some packets being received.
> Anyway, I had one of these episodes yesterday and started to play about 
> with "stuff I know little about" the first of which was fragmentation and 
> bingo, the connection came back up immediately.
> The device works almost perfectly under windows (but when its run from 
> windows its for a short time and with less load than when its in linux).
> So following this (possibly temporary) success, I'm looking into the "stuff 
> I know little about". Has anyone else had any experiences with adjusting 
> things like fragmentation, turning on RTS/CTS stuff? what values have you 
> used and what have the results been?

In most cases this kind of problem is caused by someone firewalling off
ICMP. Even some major sites like Hotmail got it into their heads at one
stage that pings could be a denial of service vulnerability and blocked
all ICMP. This contributed to the end of slip, because slip's default
MTU size was 296, and when people switched to ppp with it's 1500 MTU
default, Hotmail mysteriously started working.

ICMP is critical to path MTU discovery among other things. In most cases
people don't notice because the MTU size is 1500 all the way through.
However, when you throw in things like PPP over ethernet, which steals
40 bytes or so from the MTU, things start going haywire.

-- 
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