Even today, this is a problem. There are other network protocols that are in use today - such as Appletalk, ATM, AX.25, DECnet, Frame Relay, IPX, and OSI - and few applications actually support them. This is also a serious problem for the future, as any research into new network protocols is greatly constrained by the lesson of IPv6: that anything not IP will not be supported by the applications people want to use, and that anything that is not supported by existing applications will encounter great difficulty gaining acceptance.
The core of the BSD sockets API, especially the actual system calls, is not tied to any particular protocol. The problems fall in two major categories: supporting APIs that are protocol-dependent, and poor programming practices that are common. There have been great advances made in fixing the networking APIs in the system libraries though the efforts of the IETF's IPng working group and the IEEE's POSIX p1003.1g (networking API) working group. New library functions, data structures, and pre-processor symbols together allow addresses and other network properties to be treated as variable-length abstract objects whose internal format can be changed without the application's involvement. But there is still a serious problem of programmer education, which in turn requires good documentation of the problems and how to solve them. To date, this documentation still does not exist.
In the NRL IPv6 implementation, standard networking applications from BSD and from the Internet were taken and modified to support protocol independence. For most applications, this was straightforward once we had done a few applications and knew what to look for. The end result was not only a conversion to allow the applications to support almost any protocol, but also a significant cleanup of the applications' code.
In this paper, I will first discuss the problems that need to be solved to make programs protocol-independent: what is wrong, why it is wrong, and how it can be done right. I will then discuss the new protocol-independent API functions and compare them with older BSD networking API functions in light of the problems that need to be solved. I will also present in more detail some additional functions that we found necessary to solve certain problems that have not yet been standardized. Specific examples from freely available networking programs will be used.