An Air India Boeing 787-800, registration VT-ANU performing flight AI-120 from Frankfurt/Main (Germany) to Delhi (India) with 249 passengers, was enroute at FL390 about 340nm eastsoutheast of Tehran (Iran) when the crew decided to descend the aircraft to FL200, turn around and divert to Tehran due to cracks of the left hand windshield. The aircraft landed safely at Tehran's Imam Khomeini Airport about 65 minutes later. A replacement Boeing 747-400 registration VT-EVA was dispatched to Tehran, resumed the flight and is estimated to reach Delhi with a delay of about 18 hours. The airline reported the windshield of the Dreamliner will be replaced in Tehran, then the aircraft will return to Delhi. The occurrence aircraft is still on the ground in Tehran about 17.5 hours after landing.