Dishwasher and Sink question

My dishwasher is hooked to my kitchen sink drain. I noticed that when I did a light cleaning mode,the part where the DW drains seems to back up my kitchen sink about 2 inches. It remains there for now 1/2 hour..is this normal, or do i have a drainage problem? If its a drainage problem where do I look for the problem. Prior to useing the DW, ordinary water had no problem draining in the sink.
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Drain needs cleaning. Not, it is not normal unless the drain is stopped up.
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Doesn't there need to be something to allow air to escape? I'm drawing a blank on the correct term.
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There needs to be a loop or air gap in the line from the DW to the drain. that is to prevent siphoning back to the DW. The sink drain should still drain in the normal manner and yes, the plumbing system needs to have a vent. But if the drain is clogged, the water will have no place to go, but up into the sink.
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