Reciprocity

Diplomatic relations, it seems, are best conducted with strict reciprocity.   The diplomatic spat between India and the US over the arrest and post-arrest treatment of Indian diplomat Ms. Devyani Khobragade has led India to withdraw privileges that have long been extended unconditionally to US diplomats in India.  For instance, India has diplomatic facilities in Nepal that US diplomats used to be allowed to use.  As reported widely (e.g., here)

The US ambassador {to India} Nancy Powell, who was all set to spend her Christmas in Nepal, was forced to cancel a scheduled trip there after the {Indian} foreign ministry refused to extend her special privileges that usually come with the job. 

Powell had notified the foreign ministry about her trip and asked for the use of the protocol lounge and other privileges like security and immigration assistance, but she was curtly reminded that all of these had been withdrawn as India had decided to deal with the US on the basis of reciprocity.
Indian Ambassadors to the US never got such privileges, but for many years India decided to allow the US these and many other perks to signal that these were accorded to a special friend.

Meanwhile, relations between the US Department of Defense and the Indian Defense Ministry continue to be excellent.