Place of Supply under GST when recipient is registered in different state

Mr A is a lawyer located in Banglore. He is performing some work on behalf of Mr. B (registered in Chennai). The work is being performed in Banglore and even the client for whom it is getting done is in Banglore. Billing is being done to Mr. B at his Chennai address. What should be considered as place of supply. Should Mr. A charge IGCST or CGST and SGST.