Advice

Things we have learned and common mistakes - todo

• The spec notes that stream ids must be increasing, but this can be easy to accidentally violate. If after frame generation, you insert the frame into a priority queue, then it's possible for a higher priority frame to move ahead. Therefore, stream ids must be generated/committed later in processing in order to ensure correct stream id ordering.

• Implementations need to make sure they are always pumping data from the underlying transport to the HTTP/2 layer, otherwise there can be hangs. This is because a sender may be blocked on a zero flow control window. If it doesn't keep reading data from the socket, it may not pick up the WINDOW_UPDATE frame that reopens the flow control window.

• Some client implementations deal very poorly if a server responds to both ALPN and NPN. The behavior here is ambiguous and not defined, as NPN was never standardized and as such this is not explicitly prohibited by the ALPN specification. When a server responds in the ServerHello and negotiates a protocol via ALPN, it must not also send an NPN negotiation with a list of protocols even if the client sends both ALPN and NPN extensions in its ClientHello.