@Gromit
//Turkey had about 10 seconds to attack the Russian plane in Turkish airspace. But they fired later when the plane was in Syrian airspace.
They are also supposed to issue a warning and give the aircraft time to remove itself, under rules previously agreed on. //
Seeing as how you mentioned the ongoing Russian incursions into UK airspace, I am surprised that you did not point out the manner in which Turkey did not fly a standard intercept with parallel flying and generally escorting them off the premises.
The radio message (as featured on the news) was indistinct, crackly and delivered in English. Even if the latter were a stumbling block (for a seasoned pilot, it would not), visual contact (and photography) means they cannot play the "we did not receive radio calls" excuse and the "we were on the wrong ATC frequency" excuse.
Turkey singularly failed to adhere to border patrol etiquette (for want of a better word: I don't imagine there is any formal, internationally recognised, 'protocol') and went straight for the trigger-happy option.
What stops this escalating is that neither side would 'win': Turkey would lose the combat aspect… eventually (much grief on both sides) and, since Putin has run a campaign to drive more marriage and children (BBC Newsnight piece) then it would seem that Russia has manpower limitations and wouldn't want the further drain arising from action in Asia Minor.