IAX2 UDP Packet Loss - Application Dependant?

PROBLEM: When calling into my asterisk server using Voicemail Main I hear an occasional audio gap?

PSTN -> My Termination Provider <- IAX2 using ulaw-> IAX SERVER
No NAT involved. Publicly addressable IPs
iax.conf: jitterbuffer=yes


  1. Running iax2 show netstats shows Local Lost packets. The sound problem coincides with an increment in the Local LOST field (incremented by approximately 6 packets). A normal VoicemailMain session (not counting DTMF related erros) is at between 1-2% Packet loss.

  2. During DTMF tone transmition iax2 show netstats also shows an increment in lost packets. The Local Lost packets is almost always incremented by 9.

  3. A regular conversation from an IAXy device via my asterisk server to my provider (where I terminate my calls) shows almost no packet loss.


  1. How accurate is the iax2 show netsats in displaying lost packets. Is it possible that the local end expects x UDP packets for a DTMF tone but only (x-9) are sent by remote and therefore recieved?

  2. The nature of UDP does not allow for flow control and therefore one may expect lost packets during bursts of data. Could anyone tell me whether there could be a relation between the type of application the call is connected to (ie. Voicemail Main) and the packet loss. I would expect packet loss to be consistant and application independent but recognize that certain applications may cause more data to be sent than others. Also, voicemail main reads prompts from the disk. Could I/O wait influence packet loss? (My system I/O is idle during the above tests).


  • Asterisk version 1.2.10
  • Linux version 2.6.17 SMP (Debian)
  • Dual AMD Athlon™ MP 1600+
  • Dual 3Com PCI 3c980C Python-T (only using one) connected at 100Full
  • udev system with ztdummy used as a timing device


  • Validated there are no errors on the eth0 interface and related managed switched port. Also no duplex mismatch.
  • No UDP recieve errors (netstat -su), which would imply that they’re not discarded by my system
  • Flood ping to hosts close to the destination host (ping disable on the actual desination host) does result in dropped packets (could be priority related)
  • Tried various MTU sizes below that max MTU allowed between us and no change in packet loss.

Any insight or direction would greatly be appreciated.

Zack Stone

I really suspect this is not a network issue. In my effort to further troubleshoot this I built another asterisk installation (a more portable one) which I could use to test this functionality from other networks.

It is an AMD-TURION ML-37 laptop w/ 2GB RAM
Debian 64bit install running 2.6.17 64bit kernel.

My latest test via the same MEDIA PATH as below, except that my IAX SERVER was on another network (again verly low latency/ no saturation) resulted in the same type of PACKET loss shown vi “iax show netstats”

My last step will be to try and find another place to terminate my call to the PSTN and see whether the issue still exists.