[asterisk-users] Packet size limit for HDLC?

[Roger Schreiter]

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.