Date: 201703
Local time of day: 0601-1200
Location: OIIX.ARTCC, FO
Light: Daylight
Aircraft: MD-11
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Descent
ATC / advisory: Center OIIX
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Cruising at FL370 in Iranian airspace; Southeast-bound on airway G-666 and somewhat south of LVA (LVA - DATUT - ELIRA - ORSAR - PEBAT - DESDI); the controller directed 'Descend to FL270 30 miles from ORSAR'. I understood this to mean be at FL270; 30 miles from (ie: past) ORSAR. I tried to input the instruction to 'ORSAR/+30' into the FMS but got an 'out of range' message (because PEBAT is only 27 miles later). As I was trying to load it; the FO said he thought the controller intended us to be at FL270 30 miles BEFORE ORSAR; which would be about 10 miles too close to comply we asked the controller to say again and received same instruction. I let the FO try inputting FL270 by ORSAR/-30 since making that would comply either way and then; started an idle/full speed-brake descent since with the communication delay and inputting problems (FO also got an out of range message); we were closer yet to the requirement.Into the descent it occurred to me that; though not commonly done; possibly the controller had meant for us to BEGIN descent at 30 miles 'from ORSAR'.  I think this hadn't occurred to me sooner because that would entail commanding a descent outside of his national airspace and jurisdiction (and hadn't cited it as a directive) and because I can't ever remember a foreign controller being so precise about when to start descent; only about when/where to BE level. Far more typical that they issue the requirement and leave the descent math to the pilot; especially outside USA.So; if the controller intended we be at FL270 at ORSAR/+30; we made it - no worries. If he intended we be at FL270 at ORSAR/-30; then his direction was impossible to satisfy even had he issued it clearly and we missed it by approximately 10 miles. If he meant for us to BEGIN descent at ORSAR/+30 then; we descended early but I doubt that for the reasons I cited. In short; we aren't sure if there was a conflict or not. We do know the instruction needed clarification and we needed to pursue that further.Poor communication: ATC issued a very vague and ambiguous directive and repeated same. I did not pursue the clarification UNTIL THERE COULD BE NO MISUNDERSTANDING. I descended on assumption before any possibility of misunderstanding was resolved. Asking ATC to 'Say again' was not enough. Better; would be asking 'do you want FL270 at 30 miles North or South of ORSAR' and to have asked clearly if this was a directive to BEGIN descent at said point.


Second narrative: [Report narrative contained no additional information.]


Synopsis: MD-11 flight crew reported they had difficulty understanding the clearance they received from an Iranian Controller and may have failed to make a crossing restriction on descent.