Jumbo Frame Support Azure VM setup

Question

We are hosting a 3CX PBX on an Azure VM. We are having some network issues to Skyetel who is our SIP provider. From what I gathering it is due to some UDP packet fragmentation. They instructed me to fill out a ticket and see if it is possible to get Jumbo Frame support enabled on our account.

Here is some information from 3CX and Skyetel on the issue
https://www.3cx.com/community/threads/azure-hosting-and-skyetel-trunk.134661/

https://support.skyetel.com/hc/en-us/articles/4410352977431-UDP-Packet-Fragmentation

Thank you for the help

Answer 1

Hello @Josh Heystek,

Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.

I understand your question about network problems with your 3CX PBX on an Azure VM using the Skyetel SIP trunk. After looking at the 3CX community thread and the Skyetel support article you provided, it appears that UDP packet fragmentation is causing the issue. This often happens in VoIP environments when large RTP packets go over the MTU limit, resulting in packet loss during reassembly.

Azure Jumbo Frame Support: Azure does not support Jumbo Frames (MTU > 1500 bytes) for virtual machines or virtual networks. The highest supported MTU is 1500 bytes, and this cannot be increased at the account or resource level. Also, Azure networking does not handle out-of-order UDP fragment reassembly, which may impact SIP providers sending large UDP packets.

Reasons for This Occurrence: Some SIP providers (like Skyetel) send UDP packets that exceed the MTU limit, resulting in fragmentation. Because Azure does not reassemble fragmented UDP packets arriving out of order, this can lead to dropped packets or degraded call quality.

Suggested Workarounds:

1. Use TCP or TLS for SIP signaling: TCP is more effective at managing fragmentation and reassembly compared to UDP. The majority of SIP providers, such as Skyetel, offer support for TCP or TLS when it comes to signaling.
2. Decrease the size of the UDP packets: Adjust your PBX or SIP trunk settings to use smaller RTP packet sizes or select codecs that produce smaller payloads.
3. Consider using a Session Border Controller (SBC) or a VPN tunnel__:__ These are capable of handling fragmentation and reassembly prior to the packets arriving at Azure.
4. Consider using SIP over TLS: This approach handles signaling over a reliable stream, which helps to prevent issues related to UDP fragmentation.

For more details, please refer below links: Create, Change, or Delete Azure Network Interfaces | Microsoft Learn

UDP Packet Fragmentation – Skyetel

Kindly let us know if the above helps or you need further assistance on this issue.

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