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Absolute OpenBSD, 2nd Edition by Michael W. Lucas

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Remedial TCP/IP

Now that you have a simplified overview of how the IP system works, let’s look at a real network protocol in some depth. The dominant transport protocol on the Internet is the Transmission Control Protocol over Internet Protocol, or TCP/IP. Although TCP is a transport protocol and IP is a network protocol, the two are so tightly intertwined that they’re generally referred to as a single entity.

We’ll start with ICMP, and proceed to UDP and TCP.

ICMP

ICMP is used to transmit routing and availability messages across the network. Tools such as ping(8) and traceroute(8) use ICMP. ICMP includes all sorts of different protocols and tools.

While some people claim that you need to block ICMP for security purposes, those people don’t ...

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