asterisk users - Sep 2006 - Dropped call question - "Maximum retries exceeded on transmission"

I am encountering an intermittent issue where some of my calls are being
dropped.  Most of the calls that are made are successful.  However, some
calls will be dropped after having been connected for some time.

Each time a call gets dropped, I get output similar to the following in
the Asterisk console:

Sep 12 18:52:36 WARNING[4620]: chan_sip.c:1835 retrans_pkt: Maximum
retries exceeded on transmission **** for seqno 1620 (Critical Response)

Sep 12 18:52:36 WARNING[4620]: chan_sip.c:1835 retrans_pkt: Hanging up
call **** no reply to our critical packet.

Does anyone have any suggestions?  I honestly don't know where to start
investigating this issue, so if anyone has any ideas they would be
greatly appreciated.

Thanks

Kohler, Jeffrey wrote:
> I am encountering an intermittent issue where some of my calls are being
> dropped.  Most of the calls that are made are successful.  However, some
> calls will be dropped after having been connected for some time.
> 
> Each time a call gets dropped, I get output similar to the following in
> the Asterisk console:
...
> Does anyone have any suggestions?  I honestly don't know where to start
> investigating this issue, so if anyone has any ideas they would be
> greatly appreciated.
Jeffrey,

That's all a bit vague (how long before it drops, what protocol, are 
there firewalls, etc...), but my first guess would be a firewall NAT 
timeout. See the "NAT Issues" section at 
http://www.voip-info.org/wiki-IAX for example (it discusses IAX rather 
than SIP, but you get an idea of the issues).

- Mike