asterisk users - Dec 2008 - Packet size limit for HDLC?

Hi,

I'm using app_pppd with a Digium-PRI-card for PPP connections.
I had some strange problems with some IP packets passing
and some not, e.g. ftp login went well, but as soon as
I tried to up- or download a file, noting was transferred.

I finally guessed, it must have to do something with the packet
size. Then I started pppd with the parameters mtu 296 and mru 296
as in further times with the analogue modems.

Then, everything went fine (for a while).


Unfortunately, PPP via ISDN is typically using a MTU and a MRU
of 1500, and I found, that some commercial ISDN routers do not
allow negotiating MTU and MRU. They insist to use a size of 1500.


Since, using CAPI or ISDN4Linux (not via asterisk), pppd is working
well with the MTU/MRU value of 1500, I assume, there is some packet
size limitation in the asterisk part (including app_pppd).

I tried to find any too small buffer or similar, but successless.


May I ask you, where do you think, the limitation does come from:
- from app_pppd (I don't think so)
- from libpri
- from chan_dahdi
- from the dahdi kernel modules
- from the asterisk kernel


Any hint is welcome!

Regards,
Roger.

ICMP is used to determine maximim packet size.  If you or the other end 
are blocking all ICMP then MTU Path Discovery will not work.  It's a 
classic newbie network admin mistake.  Symptoms of this problem would be 
exactly like you describe.

Typically I see this on PPPoE connections.

More info: http://www.znep.com/~marcs/mtu/

Roger Schreiter wrote:
> Hi,
> 
> I'm using app_pppd with a Digium-PRI-card for PPP connections.
> I had some strange problems with some IP packets passing
> and some not, e.g. ftp login went well, but as soon as
> I tried to up- or download a file, noting was transferred.
> 
> I finally guessed, it must have to do something with the packet
> size. Then I started pppd with the parameters mtu 296 and mru 296
> as in further times with the analogue modems.
> 
> Then, everything went fine (for a while).
> 
> 
> Unfortunately, PPP via ISDN is typically using a MTU and a MRU
> of 1500, and I found, that some commercial ISDN routers do not
> allow negotiating MTU and MRU. They insist to use a size of 1500.
> 
> 
> Since, using CAPI or ISDN4Linux (not via asterisk), pppd is working
> well with the MTU/MRU value of 1500, I assume, there is some packet
> size limitation in the asterisk part (including app_pppd).
> 
> I tried to find any too small buffer or similar, but successless.
> 
> 
> May I ask you, where do you think, the limitation does come from:
> - from app_pppd (I don't think so)
> - from libpri
> - from chan_dahdi
> - from the dahdi kernel modules
> - from the asterisk kernel
> 
> 
> Any hint is welcome!
> 
> Regards,
> Roger.
> 
> 
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Hi,

I figured out, that app_pppd suffered from
overruns under "high" out traffic.
("ping -s 600 <destip>" was already high in this context.)

I've just made a quick and dirty hack to fix it.
If interested, just download the original package
by Sirrix (as mentioned on VoIP-Wiki) and the replace
their app_ppp.c by:

http://planinternet.net/download/voip/asterisk/app_pppd.c


Maybe I will later find the time to bundle a complete package,
like the one by Sirrix.

Regards,
Roger.