To further motivate the need for bi-directional connectivity across heterogeneous address spaces, let us consider the DSL service at CMU. In April 1999, CMU began offering an internal DSL service that allowed users to obtain as many IP addresses as needed. Twenty months later, the 2000 IP addresses allocated to the service were exhausted. To conserve IP addresses, today only one statically assigned and one dynamically assigned IP address is provided per DSL line.
The situation has driven many of our DSL users to begin using NAT to get around the address allocation problem. Unfortunately with NAT, bi-directional connectivity is lost. This drastically affects the user's computing activities because fundamentally the university environment is not a pure client-server environment and bi-directional connectivity is critical. Although the DSL user will still be able to browse the web from home and access campus computing resources, she will not be able to remote login directly to her home computers using ssh or telnet. She also will not be able to host her own web servers or ftp servers on her home computers to distribute documents like digital videos and photos. When she is accessing campus computing resources from home, she also will not be able to bring up X Windows applications on her home computers (unless ssh X Windows connection forwarding is used). Many popular peer-to-peer applications also break down. For example, when both parties are behind NAT gateways, the popular music sharing software Napster will not work.
In Section 7, we discuss a simple port number forwarding work-around that can partially address these problems. However, this work-around works in the transport layer, requires per application manual configurations, and the connectivity achieved is unacceptable as only one home computer per port number can accept in-bound connections. In contrast, as we shall see, AVES is capable of allowing DSL users who deployed NAT gateways to fully regain all the above lost capabilities.