The Maximum Transmission Unit (MTU) can be a make or break setting on your customer's Networking Equipment. In most cases, MTU will be set automatically, but if you run into issues, you may need to adjust the value manually.
TIP - If you have issues, we recommend an MTU value of 1500, which is the maximum allowable by Ethernet. This will ensure that all SIP packets are allowed to pass through (based on packet size).
Brief Explanation of MTU
The Maximum Transmission Unit is the largest size packet (in bytes) that your customer's Networking equipment can process. You may or may not be familiar with this setting because it is often set automatically, but in some cases, it can cause issues. You will find that most SIP packets will be between 500-1200 bytes, but if you allow additional Codecs or extra headers are sent more bandwidth will be required.
Our PBX does not support Fragmented UDP packets, so in the case, the MTU value is too small (compared to the actual packet size), the call will fail.
Quite commonly video calls will fail due to oversized packets.
Where do you adjust MTU?
MTU can be set on Most Network Equipment. It is important to remember that the Network Path will be limited by the lowest MTU value.
What are the Symptoms?
- Outbound call does not complete
- Inbound Call does not reach the phone
What are not Symptoms?
- Call Quality Issues - RTP Packets will be well under the average MTU size.
- Failed Parking or Transfers - Because these don't contain SDP, it's not likely to fail due to an MTU Issue
How can I tell if the call failed because of an MTU Issue?
MTU related failures can be seen in a packet capture (where to take the capture will depend on the source of the call).
If your phone fails to call outbound; you can take a packet capture from the phone. If you examine the SIP Packet in Wireshark, you can see the size of the SIP packet in the packet details pane.
If the call fails inbound, you can look at a packet capture of the call, or look at a WAN side packet capture of your customer's Firewall/Router.
TIP - If modifying the MTU size doesn't work, try connecting your device over TCP instead of UDP.