The Prime Minister of Canada Mr. Justin Trudeau said India violated Vienna Convention on Diplomatic Relations by sending back 40 diplomats from their High Commission in India back to Canada
Lets look at the provisions in 'Vienna Convention on Diplomatic Relations' for sending back Diplomats or Ambassadors from Diplomatic Missions by Receiving Country
Article 9 of Vienna Convention on Diplomatic Relations says:
The receiving State may at any time and without having to explain its decision, notify the sending State that the head of the mission or any member of the diplomatic staff of the mission is persona non grata (an unacceptable or unwelcome person) or that any other member of the staff of the mission is not acceptable. In any such case, the sending State shall, as appropriate, either recall the person concerned or terminate his functions with the mission. A person may be declared non grata or not acceptable before arriving in the territory of the receiving State.
Article 11 says:
1.In the absence of specific agreement as to the size of the mission, the receiving State may require that the size of a mission be kept within limits considered by it to be reasonable and normal, having regard to circumstances and conditions in the receiving State and to the needs of the particular mission.
2.The receiving State may equally, within similar bounds and on a non-discriminatory basis, refuse to accept officials of a particular category.
India's External Affairs Minister Dr. S. Jai Shankar clarified that there were no violation of Vienna Convention on Diplomatic Relations in the matter of sending back 40 Diplomats of Canada, since India has only 20 Diplomats in Canada and Canada holds 60 Diplomats in India, India just brought down the number of Diplomats to maintain parity.