Chirpstack-Gateway-Mesh component fails to send class C downlinks.

[vish022091]

As mentioned in the Chirpstack community, the OS fails to send downlinks to a class-C type end node through the mesh relay gateway. Here are the errors that I see when sending a downlink:

On the border gateway:

chirpstack-gateway-mesh[2577]: Handle command error: Only Delay timing is supported

On the relay gateway:

ERROR [chirpstack_gateway_mesh::backend] Handle mesh event error: Tx Ack error: TOO_EARLY

I was wondering if there's any fix to this besides the customizing the entire OS image.

Having this fixed will definitely have far reaching consequences (no pun intended) since many users would likely use the mesh framework to extend coverage when controlling class C devices like relay controllers and switches - where downlinks play a vital role.

[vish022091]

Just bumping this in case anyone has the same issue or has been able to solve this?

[j9archer]

Yes - I have the same issue. I tried the fix from Elphmatt linked in your original message, but found it needed some mods to work with my setup. Here's what I ended up doing in case it might be useful for somebody (apologies for the chatGPT summary, but the code is not worthy of a PR):

mesh.rs — What changed for Class C

Logic now dynamically chooses Immediate vs Delay timing based on the uplink metadata’s delay field:

1. Downlink timing

Old behaviour:
Everything was treated as Class A, using DelayTimingInfo.

New behaviour:

If pl.metadata.delay == 0 → treat it as Class C

Uses gw::timing::Parameters::Immediately(gw::ImmediatelyTimingInfo {}).

Else → Class A

Uses the normal DelayTimingInfo.

This inserts the correct Lorawan timing mode into the DownlinkTxInfo.

2. Downlink context handling (important for mesh relays)

Old behaviour:
Always fetched the uplink context (get_uplink_context(uplink_id)).

New behaviour:

Class C (delay == 0):
No uplink context is needed. The context field is set to an empty vector.

Class A (delay > 0):
Behaves as before: retrieves stored uplink context.

This avoids bogus context being attached to “immediate” Class C downlinks.

packets.rs — What changed for Class C

DownlinkMetadata originally encoded delay as (b[5] & 0x0F) + 1, which meant delay was never zero.

New behaviour:

Extract delay_raw = b[5] & 0x0f

If delay_raw == 0, keep it as 0
→ This now maps cleanly to “Class C immediate”

If delay_raw > 0, use delay_raw + 1
→ Normal Class A mapping preserved.

This decoding change is what enables mesh.rs to detect Class C downlinks.

Net effect

Together, these changes mean:

A zero delay value in downlink metadata now signals a Class C downlink.

Class C downlinks:

Use Immediate timing (ImmediatelyTimingInfo)

Skip uplink context

Class A downlinks:

Continue using Delay timing

Handle context exactly as before.

Caveats:

I started testing this and ended up putting it to one side, as I had the option to avoid using Class C altogether. One issue was that I had a lot of uplinks on my mesh, each of which is mesh-wrapped and tx'd from the relay gateway. So there's a lot of tx traffic at the relay gateway, and my gateways being non-duplex resulted in a bunch of missed downlinks coming from other gateways, as they can't send and receive at the same time.
I could have got around it by reducing some of the uplinks from the devices, and re-issuing unconfirmed downlinks in case they were missed, but ended up doing it differently.

[vish022091]

Thanks for sharing. I'm no dev./comp. engg. - it's def. beyond the realm of my expertise to be able to mod the source file and make my own ipk. I think it's worth a PR considering how fundamental this to class C device functioning.

My setup involves half-duplex gateways too and so I'm faced with the exact same dilemma - lots of uplinks and missed/slow downlinks. I was considering going full duplex but it's so pricey to install that many of them over a mesh.

[brocaar]

This was not clearly mentioned in the documentation, but currently this is only intended to work with Class-A.

[vish022091]

Hoping for a new update soon that supports class C mesh downlinks