Source: OECD (2019) Unlocking the digital economy - A guide to implementing application programming interfaces in government, OECD, Paris https://www.oecd.org/tax/forum-on-tax-administration/publications-and-products/unlocking-the-digital-economy-guide-to-implementing-application-programming-interfaces-in-government.pdf Establish or connect to a 4 corner connectivity model (like GSM networks or domestic mail services) Similar to the eDelivery model (service metadata locator, service metadata provider, connector, ...)