[asterisk-users] Packet size limit for HDLC?

[Eric "ManxPower" Wieling]

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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