[asterisk-users] Voice broken during calls (again...)

Marek GreŇ°ko mgresko8 at gmail.com
Tue Jun 23 02:28:34 CDT 2020


if you need clampmss then it is highly probable there is a PMTU
discovery problem. The clampmss does not work for UDP.

I probably counted the size incorrectly. So you are able to ping with
size 1464 and not with 1466. How about trying same ping sizes from the
internet towards your site? I mean trying to ping from sites with
higher MTU than yours without lower MTU links in the path.

You know MTU is a size of l2 frame, so using ipv6 you are able to use
higher payload sizes because of ip header size.


2020-06-23 9:06 GMT+02:00, Luca Bertoncello <lucabert at lucabert.de>:
> Am 23.06.2020 08:43, schrieb Luca Bertoncello:
> And another thing, I discovered right now...
>> Could you suggest me something to restrict the problem?
>> Currently, I think the problem can be:
>> 1) on Asterisk
>> 2) on my Gateway/Firewall
> A couple of years ago I added this entry in my firewall:
> /sbin/iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS
> --clamp-mss-to-pmtu
> since I had the problem downloading data from an Internet site using my
> tablet.
> I found this site explaining that:
>     https://lartc.org/howto/lartc.cookbook.mtu-mss.html
> I really forgot this entry, but now I checked all entries in my
> Firewall, and I see it, with my remark...
> Now, the last line of the HowTo:
> --------------------------------
> # iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss
> 128
> This sets the MSS of passing SYN packets to 128. Use this if you have
> VoIP with tiny packets, and huge http packets which are causing chopping
> in your voice calls.
> --------------------------------
> Could it be the problem? Right now I'm not at home, so I cannot test it,
> but maybe I can add an entry like:
> iptables -A FORWARD -p tcp -m multiport --ports 5060,<my high port for
> SIP> --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 128
> and change the previous entry like:
> iptables -A FORWARD -p tcp -i intlan0 --tcp-flags SYN,RST SYN -j TCPMSS
> --clamp-mss-to-pmtu
> to limit the behaviour on the internal LAN...
> Your opinion?
> Thanks a lot!
> Luca Bertoncello
> (lucabert at lucabert.de)
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