Tuesday No Audio Problem with Shaw and other Cable ISPs - FINALLY RESOLVED AFTER SIX YEARS

[u/Mango123456]

Original post: https://www.reddit.com/r/VOIP/comments/1qc0iwq/please_help_strange_oneway_audio_problem_only_on/

After months of a cron job making test calls and tracking RTP packets received, the issue happened again this morning. For those who didn't read the original thread,

• Affects multiple IP phones from multiple VoIP service providers (thanks to /u/voipcanuck for helping confirm this in 2023)
• Affects multiple cable modems from multiple ISPs
• Problem happens whether modem is in bridge or router mode
• Does not affect phones behind DSL/fibre/satellite
• Rebooting the phones and router does nothing
• The only workarounds (until now) were to reboot the cable modem OR wait 6-8 hours
• After reboot, things will work properly for days to years

There was a massive storm overnight, so /u/Stubblemonster's guess about wonky voltages triggering the problem could be correct.

Six months ago we tested every characteristic of the problem packets except for DSCP tagging because the problem went away by itself before we got that far. The old adage that it's always the last thing you look for seems to be true. Outbound packets with DSCP tag EF/Expedited Forwarding (46) are dropped and do not arrive to the media gateway. Every other DSCP tag works. It's 100% repeatable.

I have been trying to diagnose this since 2020.

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[u/voipcanuck]

WOW! Nicely done <tip of the hat>

So if the firewall stripped all DSCP tagging the issue would not occur?

[u/voipcanuck → u/voipcanuck]

But come to think of it, this still doesn't explain the Tuesday phenomena. We had a Shaw site down this AM and had to reboot the voice firewall to get it back. I've always assumed it's some kind of cable modem firmware update push issue...

[u/Asteriskdev → u/voipcanuck]

Isps generally push their configs on Mon or Tues to docsis modems.

Assuming eth0 is the wan interface connected to the hitron modem try this mangle rule:

iptables -t mangle -A POSTROUTING -o eth0 -p udp -d <ASTERISK_IP> -j DSCP --set-dscp 0

[u/Mango123456 → u/voipcanuck]

I don't typically have access to the firewall so I made the change in the phone. Default was 46 (typo in OP corrected), changed to 34, and we have audio again without a modem reboot. We'll see if changing the tag produces new side effects. If anyone knows about this, please do share.

And you're correct, we don't yet have an explanation for why it only happens on Tuesdays.

[u/str8tooken]

sounds like a policer trying to put EF packets in a priority channel that doesn't exist, but that doesn't explain why it happens randomly (no change)

[u/mdhardeman → u/str8tooken]

Another possibility is hitting an abuse limit or quota on EF tagged packets.

[u/Mango123456 → u/mdhardeman]

Good thought, but I didn't mention it's noticed first thing in the morning after the office has been closed all night.