Maximum Transmission Unit [MTU]

What is MTU or Maximum Transmission Unit and how will it affect your Networking Equipment?

The Maximum Transmission Unit (MTU) is the largest size packet (in bytes) that your 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 to be sent over your network, more bandwidth will be required.

Our PBX does not support Fragmented UDP packets, so in case if the MTU value is too small (compared to the actual packet size), the call will fail. Voradian MobileReach and Video phones will often have packets that approach the value of 1500. 

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 from RTP-Sight, or look at a WAN side packet capture of your Firewall/Router.

Your network administrator or IT department can help set the correct MTU value for your network. If you suspect a MTU related issue, you can also send us a trouble ticket. We will do our best to help you direct in the right path to resolve your network issue.

If the call fails inbound, you can look at a packet capture of the call from RTP-Sight, or look at a WAN side packet capture of your Firewall/Router.

NOTE:

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).