"Apple appears to be using the HTTP/2 protocol only as a mechanism to maintain persistent connections, passing empty HEADERS frames to open new streams. This breaks section 8.2.1 of the HTTP/2 specification. We work around this by modifying the h2 module to allow empty HEADERS frames.

Apple’s use of the HTTP/2 protocol is also irregular for two other reasons. First, the HTTP/2 specification mandates that stream IDs must be created in a monotonically increasing order. However, Apple’s application endpoints appear to hardcode a handful of particular stream IDs for certain communications, which when opened do not conform to this monotonic ordering requirement. Second, client-server communication patterns often use multiple streams in unusual ways. For instance, heartbeat messages send requests over stream 1 and responses on stream 3."