So I'm perfecting my notes here and I wanted to clarify a few things, make sure I have it right. I know that if a port scanner or something is checking a TCP port it will send TCP syns for the port and will try to establish a 3 way handshake. For UDP I know that it will just send protocol data depending on the port, or random data hoping for a response or icmp type 3 code 3. However, I want to be sure about one thing. Is it absolute that a TCP host SHOULD send a TCP RST, or can they send an ICMP type 3 code 3 as well? Is this just how the implementations have gotten and that's now the norm (tcp using tcp rst and udp using icmp type 3 code 3?) Is there ever a scenario when a host rejecting a TCP connection would send a icmp type 3 code 3?
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