Is it possible to initiate http2 session or stream from Jetty Assuming a http2 connection already exists?

Question

It is possible to do server push. But if the client is the low-level jetty client, is it possible to initiate a new session or stream from the server? the assumption is the client is low-level jetty based client and the connection is already established.

Answer 1

After the initial connection is established, and the prefaces exchanged, HTTP/2 is a symmetric protocol.

The HTTP semantic requires the client to initiate requests, but at the lower level - at the HTTP/2 protocol framing level - this is not necessary and it is possible for a server to initiate a stream towards a client.

While the HTTP/2 protocol framing is symmetric after the preface, it is still tied to the HTTP protocol semantic , that is you need to send a HEADERS frame (even an empty one) before a DATA frame. However, this may not be of much hindrance if you want to build your own protocol on top of the HTTP/2 framing, you will just have few additional bytes to send over the network.

As an aside, there are proposals that use the HTTP/2 framing to transport WebSocket (a pure bidirectional protocol) frames inside HTTP/2 DATA frames, in what is essentially an infinite request with an infinite response. But I digress.

As for the Jetty specific implementation of HTTP/2, is it possible to initiate a stream from the server towards a client in Android ?

The answer is two-fold.

The first is that the current implementation (Jetty 9.3.8) has some assumption that the protocol being transported by the HTTP/2 framing is HTTP. As such, a server-initiated stream is currently dropped by the client. It would be fairly easy, though, to override this behavior and allow the client to properly handle the server-initiated streams, in the same way the server handles client-initiated streams.

The second is that Jetty's HTTP/2 support in general requires JDK 8, and at this time this is not available in Android. If there already exist HTTP/2 Android clients that are capable of handling server-initiated streams, please comment on this answer which one, as I am really interested.

The idea of server-initiated streams is intriguing though, and I filed this issue to keep track of it. If this is really important to you, you can contact Webtide (the company behind Jetty) to sponsor the implementation.