I think Jan makes a good point.

Suppose there’s a server that supports DNS over TCP, and DSO signaling, and 
Push Notifications, and DNS Update, and maybe other things.

Now suppose a client connects to that server. The server doesn’t know what that 
client is going to do. The client may do queries over TCP, or DNS updates. It 
may do queries over TCP and use the DSO signaling to request a longer 
inactivity timeout. It may request Push Notifications (which are currently 
specified to require TLS). It may do all of those.

When the server receives an incoming TCP connection request from a client, what 
are the first bytes received over that TCP connection? Are they a DNS header 
and message body? Are they a TLS handshake message? Can it be either? How does 
the server know?

Stuart Cheshire

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