Interesting question and I guess it depends what you mean by "protocol"[1]. If simple characters-over-the-wire counts, then ASCII was developed throughout 60s and was itself derived from much older telegraph standards. Teletype is arguably more of a "protocol" and is actually even older, going back to the 50s, and arguably the 30s in some form, and still forms the basis of all terminal computing today including (especially) across networks.

[1] Sometimes it seems like every interesting discussion boils down to definitions, doesn't it?