Résumé : Grid computing applications are challenged by current wide-area networks: firewalls, private IP addresses and network address translation (NAT) hamper connectivity, the TCP protocol can hardly exploit the available bandwidth, and security features like authentication and encryption are usually difficult to integrate. Existing systems (like GridFTP, JXTA, SOCKS) each address only one of these issues. However, applications need to cope with all of them, at the same time. Unfortunately, existing solutions are often not easy to combine, and a particular solution for one subproblem may reduce the applicability or performance of another. In this paper, we identify the building blocks that are needed for connection establishment and efficient link utilization. We present an integrated solution, implemented within the Java-based Ibis runtime system. Our NetIbis implementation lets applications span multiple sites of a grid, and copes with firewalls, local IP addresses, secure communication, and TCP bandwidth problems.