VOIP connection not established in demo code

[appbasehamburg]

In the current version of the matrix-js-sdk the VOIP demo code can't establish a call between two room members.
(https://github.com/matrix-org/matrix-js-sdk/tree/develop/examples/voip)
The incoming call is recognized and even the call answering event is handled correctly, but the remote streams are not connected successfully.

Steps to reproduce the behavior:

1. Checkout the voip demo code: https://github.com/matrix-org/matrix-js-sdk/tree/develop/examples/voip
2. Setup two demo accounts on matrix.org
3. Connect them and chat on the same, unencrypted room
4. Add the credentials and the room-id to the demo code
5. Start a demo web server providing this code
6. Use two clients (different browsers, different clients on the same network..)
7. Open the web demo on both using the two different users
8. Call the other member using the provided buttons

Expected behavior
The call is displayed on the receiver side and even the answering works. Both clients are now in an active call state, but the streams are never connected.

Tested on an iPad using Firefox and Safari, on a Samsung Galaxy S21 Ultra using Chrome and Firefox, on a Macbook Pro 2020 using Safari, Chrome and Firefox

Additional context
We started downgrading the library step by step to see, if this issue came up recently.
Tested versions that don't work:

• 15.3.0
• 15.0.0
• 14.0.1
• 12.5.0
• 12.3.0
• 12.3.0-rc1
• 9.8.0

Tested version that do work:

• 11.0.0
• 12.0.0
• 12.2.0

The connection issue appeared in version 12.3.0-rc1. Version 12.2.0 is the last (released) working version.

Edit: Tested versions added

[SimonBrandner]

Will investigate over Christmas

[SimonBrandner]

I can confirm the calls work between Element Web <-> demo code but no between demo code <-> demo code

[appbasehamburg]

I don't know, if the hint is helpful, but every client (Android Element / iOS Element / Element Web) <-> Demo Code works

[SimonBrandner]

Ha! This has an actually interesting cause - we use the MatrixClient::deviceId as ourPartyId but for the demo, it's not set, so it just sends null which breaks answering calls

[appbasehamburg]

I just tried to use the deviceId in your demo code. We adjusted the code to use the login method and username + password with demo accounts on matrix.org.

var client = matrixcs.createClient({
    baseUrl: BASE_URL,
    timelineSupport: true,
    deviceId: "web-demo",
})

const login = (userNo) => {
    client.login("m.login.password", getExternalMatrixTestLogin(userNo)).then((resp) => {
        if (resp.user_id) {
            console.log("Login succeeded - starting client")
            client.startClient()
        } else {
            console.log("Login failed", resp)
        }
    })
}

where getExternalMetrixTextLogin(userNo) returns and { user: xxx, password: xxx }

Using the deviceId as you mentioned doesn't fix the issue in our case.

[appbasehamburg]

Okay it's working now: The deviceId must not be the same for both clients. I just randomized it and now the streams connect and the call is working flawlessly.

[SimonBrandner]

Okay it's working now: The deviceId must not be the same for both clients. I just randomized it and now the streams connect and the call is working flawlessly.

Yes, it must be different for all devices :)

[appbasehamburg]

Just to clarify: ALL devices in a group chat must have different ids? Isn't there a high possibility, that there are more devices using the same id (like "iPhone", "web", "Android"...) in the same room?

[SimonBrandner]

Just to clarify: ALL devices in a group chat must have different ids? Isn't there a high possibility, that there are more devices using the same id (like "iPhone", "web", "Android"...) in the same room?

The spec says this: https://github.com/matrix-org/matrix-doc/blob/dbkr/msc2746/proposals/2746-reliable-voip.md#add-party_id-to-all-voip-events

[appbasehamburg]

@SimonBrandner The spec just says the partyId should be unique for call events, but is using the deviceId as the partyId implicitly a good idea?