What did Russia promise Syria after the Il-20 loss?

What did Russia say it would do after one of its military aircraft was shot down over Syria? By 29 September 2018, the answer had been public for several days.

The background was the night of 17 September 2018, when the Israeli Air Force struck multiple targets in government-controlled western Syria. Syrian air defences responded and accidentally shot down a Russian military Il-20, killing all 15 crew aboard. Russia blamed Israel, accusing the air force of hiding behind the Russian aircraft and so putting it in the line of fire. The Israel Defense Forces acknowledged striking a Syrian weapons facility and expressed sorrow for the loss of life caused by Syrian fire.

The strikes came a few hours after Russia and Turkey agreed to create a demilitarised zone around Idlib Governorate, an arrangement that postponed an imminent offensive by Syria's forces and its allies.

On 24 September 2018, Russian officials announced their response. They said the S-300 air-defence missile system would be sent to Syria within two weeks of the downing. They also said Russia would begin jamming radar and satellite signals of aircraft attacking Syrian territory from the Mediterranean.

By 29 September 2018, those announcements marked the Russian position on the aftermath of the Il-20 incident: a pledge of stronger air defence and electronic interference, set against Moscow's accusation that Israel bore responsibility for the loss of the aircraft and its crew.