io.grpc uses a copy of OkHttp 2.7.5
a few of the updates from the changelog since then:
3.12.0: OkHttp now supports TLS 1.3. (required for Android 10)
3.14.0: OkHttp now supports duplex calls over HTTP/2.
4.1.0: Fix: Recover gracefully when a coalesced connection immediately goes unhealthy.
4.1.1: Fix: Don’t drop repeated headers when validating cached responses.
4.2.0: Fix: Acknowledge and apply inbound HTTP/2 settings atomically. Previously we had a race where we could use new flow control capacity before acknowledging it, causing strict HTTP/2 servers to fail the call.
4.2.0: Fix: Don’t lose HTTP/2 flow control bytes when incoming data races with a stream close. If this happened enough then eventually the connection would stall.
4.2.1 is what we currently want to target.