ASIC Offloading & VoIP Quality Issues

Problem Description

Real-time VoIP applications (Discord, Teams, Zoom) exhibit high latency (>200ms) and packet loss, forcing a fallback to low-quality TCP relays. Audio input appears "damped," robotic, or near-silent to other participants, despite sounding perfect in local monitoring or DAW software.

Context This issue occurs in environments with two specific conditions:

Network: A "Double NAT" setup where a corporate firewall (e.g., FortiGate) sits behind an ISP router (e.g., FritzBox).
Audio: A multi-channel professional audio interface is used, but the microphone is connected to inputs other than channels 1 & 2 (e.g., Rear Line Inputs).

Solution 1: Network Configuration (Fixing Latency)

In Double NAT scenarios, standard firewall behavior often randomizes UDP source ports ("Port Scrambling"), breaking the peer-to-peer handshake required for low-latency VoIP. Additionally, hardware acceleration chips (ASIC/NPU) may drop these complex UDP packets.

For FortiGate (FortiOS 7.4+): You must modify the firewall policy governing the PC's traffic to force CPU processing and preserve the original source port.

Disable Hardware Offloading: Prevents the NPU from dropping fragmented UDP return traffic.
Enable Fixed Port: Forces the firewall to use the same internal source port for the external connection, allowing the upstream router to maintain the session.

CLI Commands:

Bash

config firewall policy
    edit <POLICY_ID>
        # Force traffic to main CPU (Software Processing)
        set auto-asic-offload disable
        set np-acceleration disable

        # Prevent Port Translation (Fixes Double NAT Handshake)
        set nat enable
        set fixedport enable

        # Disable Deep Inspection for Real-Time Traffic
        set utm-status disable
        set ssl-ssh-profile "no-inspection"
    next
end

After applying, clear active sessions to force a fresh handshake:

Bash

diagnose sys session filter dport 50000 65535
diagnose sys session clear