CTCL, short for computer-to-computer link, is a term from the Indian securities market. It refers to the facility offered by the National Stock Exchange of India (NSE) under which a trading member may connect its own software, or software bought from an approved vendor, directly to the exchange's trading system, in place of the trading terminal supplied by the exchange. The link is an application programming interface between the broker's systems and the exchange. The Bombay Stock Exchange offers a comparable facility under a different name.
CTCL is the base on which Indian brokers built internet and mobile trading for clients, and later direct market access and algorithmic trading. Its use is subject to conditions set by the exchange and by the regulator, the Securities and Exchange Board of India (SEBI). These cover approval and testing of the software, registration of terminals and users, and order-level checks in the broker's risk management system.