Bumping this down to a P3. It'd be nice to get this in milestone beyond M53

I think we would need this API on both Android and iOS.

Does it make sense to add this as a parameter for CronetEngine, or do we need it on per-UrlRequest basis? 

Do I understand correctly that the initial window size is only used when H2 session is established?

The initial window size advertised by the client determines how much data the server can send on a newly opened stream before waiting for a window update. The initial window size is usually only negotiated at the beginning of a HTTP2 session, but it is used for every new stream.

If Cronet does not provide users a way to set the initial window size or otherwise update flow control, then HTTP2 peers will only be able to send 64kB of data at a time, even if the BDP of the connection is much higher.

Agreed - this is likely needed on CronetEngine. Arguably you'd want to be
able to specify on a per-origin basis but also OK with changing the default
globally.

Chromium currently has a session-level window size of 15 MB and a stream-level window size of 6 MB.  It is defined at [1], which sets HttpNetworkSession::Params members, which get passed to SpdySessionPool, which get passed to SpdySession, which sends out the SETTINGS_INITIAL_WINDOW_SIZE value in the initial SETTINGS frame for the stream level window size [2], and sends a WINDOW_UPDATE frame for the session level window size [3], both in SendInitialData().  Also see the spec [4] for more details.

I believe these values guarantee that network bandwidth-delay-product is fully utilized, so I am closing this bug as WontFix.  Some autotuning of window sizes would be desirable, but that's beyond the scope of this issue.


[1] https://cs.chromium.org/chromium/src/net/http/http_network_session.cc?q=kSpdySessionMaxRecvWindowSize
[2] https://cs.chromium.org/chromium/src/net/spdy/spdy_session.cc?q=SETTINGS_INITIAL_WINDOW_SIZE&l=2716
[3] https://cs.chromium.org/chromium/src/net/spdy/spdy_session.cc?q=IncreaseRecvWindowSize&l=2730
[4] http://httpwg.org/specs/rfc7540.html#InitialWindowSize