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IMO the OSI layer system (even though using TCP/IP suite) has some merit in education. To most of us, the concept of layering protocols may seem obvious, but I'

IMO the OSI layer system (even though using TCP/IP suite) has some merit in education. To most of us, the concept of layering protocols may seem obvious, but I've talked to people who are just learning this stuff, and they have a lot of trouble understanding it. Emphasizing that each layer is (in theory) cleanly separated and doesn't know about layers above and below, is a very useful step towards understanding abstractions.

ahoka 2y ago
The problem is that this is not true. There are no such clean strictly hierarchical layers in most of the protocols that make up the internet.
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- kragen 2y ago
  There are; VNC doesn't know about IP, for example, and TCP doesn't know about Ethernet. IP is just as happy to run over PPP or Wireguard as over Ethernet. HTTP/1 knows about TCP/IP, but only a little bit, and you can easily run HTTP/1 over other protocols like TLS. Character-cell terminal protocols know very little indeed about the protocol layer under them and work almost equally well over telnet, rsh, SSH, a serial port, a modem, or a bare pseudo-TTY, the main surviving exception being window resize handling. The problem is that ① the layers don't have a fixed relationship to each other the way the OSI model proposes, ② several of the OSI layers don't exist at all in real-life TCP/IP protocols, and ③ there are other layers in current stacks that have no analogue in the OSI model, like Wireguard, MPLS, SSH, TLS, and HTTP. If you want to understand the services HTTP provides to the protocols that ride on top of it, you need to read Roy Fielding's thesis, not X.225.
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