More and more of our clients are turning to us for help with IP networking — so we’re currently publishing a series of articles where we focus on key lessons and strategies from the front lines of IP and VoIP convergence.
Recently, one of our clients found themselves stuck. They were trying to set up a SIGTRAN connection to a remote STP (Signal Transfer Point) through a FortiGate firewall, but no matter what they tried, the connection just wouldn’t come up.
They’d already worked with their Managed Service Provider (MSP) to try to resolve the issue. But while the MSP was perfectly competent with general IT networking, they weren’t familiar with the quirks of SIGTRAN or the SCTP (Stream Control Transmission Protocol) transport layer that it relies on. Their attempts at configuration fell short, leaving the client frustrated and disconnected.
That’s when they reached out to us.
SCTP isn’t your typical protocol
Unlike more familiar protocols like TCP and UDP, SCTP is relatively rare outside of telecom networks. It was specifically designed to support signaling protocols like SS7 over IP — and in this case, it was being used to carry SIGTRAN messages between the client’s voice switch and a remote STP.
The firewall, unfortunately, didn’t know what to make of it.
When performing NAT (Network Address Translation), most firewalls will assign a new source port for outbound connections. This is usually fine for web traffic, VoIP, and general data. But SCTP can be fussy — many remote STPs expect the source port on incoming packets to match what they expect, and if the NAT process changes it, the connection simply fails to establish.
The magic flag
Because we’d worked on similar projects in the past, we knew exactly where to look. Fortigate firewalls (like many enterprise firewalls) offer advanced options for handling less common protocols, including the ability to preserve the source port when performing NAT.
With the right flag enabled, and the firewall rules adjusted to allow SCTP traffic through, the SIGTRAN connection came up cleanly. Problem solved.
Bridging two worlds
This story is a great example of why deep telecoms expertise still matters, even in projects that appear to be “just” network configuration. Generalist IT providers often lack the protocol-level understanding needed for voice-specific systems, and that can lead to delays, outages, or lingering misconfigurations.
Our team brings a unique blend of IP networking know-how and telecom protocol experience to the table. Whether you’re running SS7 over SIGTRAN, handling VoIP NAT traversal, or configuring VLANs for your voice traffic, we know how to make the pieces work together.
So if you ever find yourself stuck between your switch and the firewall, you know who to call.
Would you like help with SIGTRAN, SCTP, or other voice-over-IP challenges? Contact us today to talk about how we can help.
