[Uncle Peter]

The Russians' prime motivation for being in this conflict is to keep Assad in power which means suppressing any ethnic or secular group that might organise themselves against him: Exactly how he's stayed in power to this date.

That means not just IS, but the Syrian Turkmen from the North Western borderlands and are vehemently opposed to Assad. Suppose then for instance that Ankara have become pretty fed up of the Russians making ethnic Turks a legitimate target under the pretense of the fight against IS thus taking the opportunity to down a Russian aircraft near the border knowing full well that the facts will be disputed until the cows come home.

Either way it will be interesting to see how the press spin this one. I doubt that the true facts will be uncovered anytime soon.