tendrils/notes/dantemcast.md

Dante Multicast Flow Discovery

Solution (2026-01-23)

Problem: Multicast flows showed ?? (dante-av:xxxxx) instead of the TX device name.

Root Cause: UpdateDanteTxChannels() was never being called, so GetDanteTxDeviceInGroup() couldn't identify which member of a multicast group was the transmitter.

Fix: Added call to t.nodes.UpdateDanteTxChannels(info.Name, ip, ...) when a device with TX channels is discovered via ARC protocol.

How it works now:

1. TX device discovered via mDNS, probed via ARC (port 4440)
2. ARC query 0x1000 returns TX channel count > 0
3. UpdateDanteTxChannels() marks the device as a transmitter
4. IGMP snooping detects TX device joining multicast group
5. IGMP snooping detects RX device joining same multicast group
6. GetDanteTxDeviceInGroup() finds the TX device (has TxChannels) in the group
7. Flow displayed as TX-DEVICE -> RX-DEVICE

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Protocol Notes

ARC Protocol Packet Formats

There are two packet formats observed:

Format 1 (0x27xx) - Used by our code for basic queries:

27 xx LL LL 13 xx CC CC 00 00 [args...]
   ^seq     ^0x1300|seq

Format 2 (0x2809) - Used by Dante Controller:

28 09 LL LL SS SS CC CC 00 00 [args...]
      ^len ^seq  ^cmd

Basic commands (0x1000, 0x1003, 0x3000) work with Format 1. Advanced commands (0x2600, 0x3600) require Format 2 to avoid error status 0x0030.

Commands Tested

Command	Purpose	Status
0x1000	Channel counts	Works
0x1003	Device name	Works
0x2000	TX channel names	Works
0x2010	TX flow slots	Returns zeros (no flow config)
0x2600	TX unicast flows	Returns zeros (multicast not included)
0x3000	RX subscriptions	Works for unicast; multicast has empty TX device
0x3600	RX unicast source	Returns zeros (multicast not included)

0x2600/0x3600 Unicast Flow Commands

These commands work for unicast flows but return empty data for multicast flows.

Query format (34 bytes):

28 09 00 22 SS SS 26 00 00 00
00 00 00 00 00 00 00 00
00 01 00 01 00 01
00 00 00 00 00 00 00 00 00 00

Successful response (from pcap with active unicast flows):

• 148 bytes with status 0x0001
• Contains MAC address pattern 08 02 10 e2 ef fd XX XX
• MAC can be correlated between TX (0x2600) and RX (0x3600) devices

Empty response (current network with multicast flows):

• 50 bytes with status 0x0001
• Data section shows 10 00 (0 records) instead of 10 01 (1 record)
• No MAC data available

Multicast vs Unicast

Unicast flows: RX device stores TX device name in subscription record (0x3000 response).

Multicast flows: RX subscription has empty TX device name. Routing is handled at network layer via IGMP. The Dante device doesn't store "which device is transmitting to me" because multicast is anonymous.

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What Works

1. IGMP correlation - When TX devices are marked via UpdateDanteTxChannels(), we can identify the transmitter in a multicast group by checking which group member has TX channels.

2. Unicast flow discovery - 0x3000 returns full TX device name and channel for unicast subscriptions.

3. Device discovery - mDNS + ARC protocol reliably discovers all Dante devices.

What Doesn't Work

1. MAC-based correlation - 0x2600/0x3600 return empty data for multicast flows. Only works when unicast flows are active (as seen in pcap from Dante Controller).

2. Per-channel multicast routing - We can identify TX device -> RX device at the device level via IGMP, but not individual channel mappings within a multicast flow.

3. 0x2010 flow slots - Always returns zeros on tested devices (AJA, Shure, A&H).

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Pcap Analysis Notes

Captured Dante Controller polling the network. Key findings:

• Dante Controller uses 0x2809 packet format
• Queries many commands we hadn't tried: 0x2320, 0x2400, 0x3300, 0x3400
• 0x2600/0x3600 responses contained MAC addresses when unicast flows were active
• MAC pattern: 08 02 followed by Audinate OUI 10 e2 ef
• Same MAC in TX's 0x2600 response matches RX's 0x3600 response = flow link

Commands observed (sorted by frequency):

• 0x1000, 0x1003, 0x1100, 0x1102 - Device info
• 0x2000, 0x2010, 0x2032, 0x2204, 0x2320, 0x2400, 0x2600 - TX queries
• 0x3000, 0x3300, 0x3400, 0x3600 - RX queries