I thought it was about proper noun vs common noun.
"an internet" is common noun – an abstract concept of which many particular implementations could exist.
"the Internet" is a proper noun – one specific implementation of that concept. Right now, the only one of any significance, but that may not last forever.
If humans someday colonise Mars – TCP/IP is not suitable for interplanetary use. We'll likely end up with two separate TCP/IP internets, one on Earth and one on Mars, with non-IP protocols used to communicate between them. The Earth-based one would be descendant of "the Internet" we are using right now, the Mars-based one would be a new internet.
(By the time we colonise Mars, I wouldn't be surprised if TCP has been wholly replaced by QUIC, so instead of TCP/IP we'd all be using QUIC/UDP/IP.)
The congestion algorithms/parameters may have to be updated, just like they were when WiFi and cellular came along, but the protocol itself is quite robust.
TCP is unsuitable for interplanetary links, due to the massive round trip times (e.g. Earth-Mars RTT is between 3 minutes and 22 minutes.) Technically TCP could work, but the performance in practice would be atrocious. Wifi and cellular aren't comparable because (when working) they don't have RTTs measured in minutes.
That's why for interplanetary use IETF has been developing an alternative protocol suite called DTN. Unlike TCP/IP, DTN is not real-time, it is based on store-and-forward of messages (conceptually similar to UUCP.)
"an internet" is common noun – an abstract concept of which many particular implementations could exist.
"the Internet" is a proper noun – one specific implementation of that concept. Right now, the only one of any significance, but that may not last forever.
If humans someday colonise Mars – TCP/IP is not suitable for interplanetary use. We'll likely end up with two separate TCP/IP internets, one on Earth and one on Mars, with non-IP protocols used to communicate between them. The Earth-based one would be descendant of "the Internet" we are using right now, the Mars-based one would be a new internet.
(By the time we colonise Mars, I wouldn't be surprised if TCP has been wholly replaced by QUIC, so instead of TCP/IP we'd all be using QUIC/UDP/IP.)