An agreed order of steps two parties both follow. A protocol lets strangers act together without trusting each other first.
Greek protokollon (the first sheet glued to the front of a papyrus roll, recording its contents): protos (first) + kolla (glue); a protocol was originally the leaf that told you what the document behind it contained, before the sense of an agreed procedure took hold.
An agreed order of steps that two parties both follow without having to trust each other first. The root's glued-on first page is a fitting ancestor: a protocol still tells everyone, in advance, what is about to happen.