Does Firefox WebTransport Fall Back to HTTP/2 When UDP Is Blocked?
On a network that blocks UDP, ordinary HTTP/2 pages load but your WebTransport connection fails. What does Firefox’s new supportsReliableOnly property actually tell you?
- Firefox 158 exposes supportsReliableOnly but returns false: WebTransport over HTTP/2 is still unsupported.
- Setting requireUnreliable to false cannot add a transport the browser has not implemented.
- Check browser capability, established-session reliability and connection failure separately.
Firefox 158 still needs an HTTP/3 path
Firefox 158 does not gain an HTTP/2 fallback for WebTransport. Mozilla’s release notes explicitly say that HTTP/2 is not supported and the newly exposed WebTransport.supportsReliableOnly property always returns false.
Applications designed for networks that block UDP therefore cannot retire an existing alternative connection merely because this property has appeared.
An exposed property can still report false
supportsReliableOnly is a static property on the class. It reports whether the browser supports sessions over reliable-only transports such as HTTP/2. It does not test connectivity to your endpoint.
This example distinguishes an unavailable API, an unavailable capability query and an explicit result:
if (typeof WebTransport === "undefined") {
console.log("WebTransport unavailable");
} else if (!("supportsReliableOnly" in WebTransport)) {
console.log("Reliable-only support cannot be queried here");
} else {
console.log(WebTransport.supportsReliableOnly);
}Firefox 158 reaches the final branch and reports false. Testing only for the property’s presence would lose that distinction.
requireUnreliable controls a requirement, not implementation support
The constructor option requireUnreliable defaults to false. Setting it to true prevents an HTTP/2 connection when HTTP/3 is unavailable. Leaving it false removes that requirement; it does not implement an absent fallback.
The server must also support WebTransport’s connection requirements. The W3C connection procedure checks relevant protocol settings and server support. An endpoint serving ordinary pages over HTTP/2 is not sufficient evidence that it can accept a WebTransport session.
Read reliability after connecting and handle failure explicitly
The instance’s reliability describes the actual session:
| Value | Meaning |
|---|---|
pending | The connection has not been established. |
reliable-only | Only reliable transports are supported. |
supports-unreliable | Both reliable and unreliable transports are supported. |
Wait for ready to fulfill before using the connection, and handle rejection. A failed attempt alone does not establish that UDP was blocked.
If the application offers WebSocket or another alternative, both client and server need that separate path. WebTransport does not automatically enter the application’s other protocol. Before changing a deployment’s fallback policy, test successful connections and failure handling on the networks the application needs to serve.