| Summary: | The present Internet is not defined in terms of its semantics, at least at the packet level. The loose packet carriage model of “what comes out is what went in” is intentionally almost semantics-free. The packets just carry bytes. Packet boundaries can have some limited semantics, but not much. The original design presumed some constraints on the semantics of packet headers, such as global addresses, but the progress of time has violated these and the Internet keeps working. TCP does impose some modest semantic constraints, but of course TCP is optional, and not a mandatory part of the architecture.
What defines the Internet, and the range of behavior that is available in the Internet, is the expressive power of the packet header, which has more to do with its format than any semantics. Most fields (e.g. packet length) are unremarkable, some (like the TOS bits) have been redefined several times in the history of the Internet, some (like the options) have atrophied, and some (most obviously the IP addresses) have had a most interesting history in which the only constants are that they are 32 bit fields, that whatever value they have at each end must remain constant for the life of a TCP connection (because of the pseudo-header) and that at any locale in the network, they must provide the basis for some router action. They can be rewritten (as in NAT), turned into logical addresses (as in multicast or anycast), and they can be microcoded in a number of ways to capture address hierarchy (net/rest, A/B/C, CIDR). All that really matters is that they are 32 bits long, and that at any point, they must have at least local meaning to a forwarding process.
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