Both forms are in common use...
It's normally entirely a stylistic choice whether to include the article or not. If a specific "protocol" has been previously mentioned, I'd say the article is more likely to be included (but it's only more likely - it's never required).
But if you want a "general principle" for making the choice here, my advice would be not to include the article unless it's contextually necessary to indicate that you're only referring to some specific contextually-identified protocol, rather than just "diplomatic conventions" in general.