Submits an ALTSVC frame (as defined by RFC 7838) to the connected client.

const http2 = require('node:http2');

const server = http2.createServer();
server.on('session', (session) => {
  // Set altsvc for origin https://example.org:80
  session.altsvc('h2=":8000"', 'https://example.org:80');
});

server.on('stream', (stream) => {
  // Set altsvc for a specific stream
  stream.session.altsvc('h2=":8000"', stream.id);
});

Sending an ALTSVC frame with a specific stream ID indicates that the alternate service is associated with the origin of the given Http2Stream.

The alt and origin string must contain only ASCII bytes and are strictly interpreted as a sequence of ASCII bytes. The special value 'clear'may be passed to clear any previously set alternative service for a given domain.

When a string is passed for the originOrStream argument, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string'https://example.org'. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A URL object, or any object with an origin property, may be passed asoriginOrStream, in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin.