Are questions about (application level) network protocols ok here?
This depends on the context. Questions should always be targeted towards solving an actual technical problem you face while administering a system. Remember, our topic is professional system administration, not software development or "Ask an expert" for things you are just curious about.
As an example, if you ask about some details in HTTP to solve an actual problem with your load balancer or something like that, this might be on topic here. If you ask because you want to write your own web server, this would be off-topic.