Find the document here: SSH-CONNECT
In "Opening a Channel" (SSH-CONNECT, 5.1), it is stated that "If the recipient of the SSH_MSG_CHANNEL_OPEN message does not support the specified 'channel type', it simply responds with SSH_MSG_CHANNEL_OPEN_FAILURE".
Does this mean that ONLY the SSH_MSG_CHANNEL_OPEN_FAILURE byte is sent, as was the case for SSH_MSG_REQUEST_FAILURE, or would the message be sent with the "SSH_OPEN_UNKNOWN_CHANNEL_TYPE" reason code, with all the same fields as the other failure messages?
Perhaps SSH_OPEN_UNKNOWN_CHANNEL_TYPE might just mean that the recipient is unaware that any such 'channel type' exists, where as the statement provided is a response to the event that the recipient KNOWS that said 'channel type' DOES indeed exist, but explicitly does not support it?