SIP and UDP Fragmentation ( Serial and Fastethernet )

[Zayar Win]

Dear VOIP Expert,

                       I have a doubt with VOIP. I have a serial link which is L3 MPLS VPN . Now, I have changed to L2 VPN  IEPL Link.

At this time, I have one problem with VOIP. I can not call to international. So I roll back to serial it is back to normal. And then I try again to IEPL link .

Still same error. So I have try several time . Still same error. In the meanwhile , I take the packet capture on my WAN Router interface.

I saw UDP Packet are being  fragmentation on IEPL link. When I change to Serial link there is no fragmentation. So, I suspect that SIP server do n't allowed the UDP fragmentation.

How can I solve this problem , My found   is  correct or not.

How can I find root cause for more specific troubleshooting.

Thanks and Regards,

[Zayar Win]

Currently I am using serial link that we called as IPLC which is L3 MPLS VPN link. I am using both data and VOIP traffic on this link. So far ok but Link bandwidth is only 2 Mbps.

So we had hire 10 Mbps link from other ISP. That link technology is totally changed , they called as IEPL L2 VPN link. On this L2 VPN link , I also use data and  VOIP traffic . It is ok but I have encountered the problem with one CUCM can't call to international call. In the meanwhile , some voip pattern is still ok. But only that CUCM can't call to international call. In our Voice gateway have only one Public IP address. In the mean while , I can call international call with other VOIP Pattern. So I don't suspect to Routing issue.

When I roll back to serial link call is back to normal. I have tried several time change to IEPL link and Serial link , it is still same error.

So I had capture the packet on both serial and IEPL link .  In the capture file, I saw  ipv4 fragmentation on IEPL link . And I saw the fact SIP-UDP can't accept UDP fragmentation . I am not sure but I found on this blog.

http://blog.csrpswitch.com/sip-udp-fragmentation-and-kamailio-the-sip-he...

Thus, I suspect this IEPL link is drop my VOIP packet.

Please let me have your opinion what should I check for my problem.

Thanks you

[Evgeny Izetov · Accepted solution]

You could try carrying SIP signalling traffic over TCP instead of UDP, like the article suggests. You would do that by configuring 'session transport tcp' on your dial-peers. It may not be supported by your service provider but worth a shot.

[Zayar Win]

This answer is absolutely correct answer. I have change to protocol to TCP instead of UDP in voice gateway.

Very much thanks  Mr. Evgency Izetov