Conclusions

1. IP dates back to RFC 791 (1981), and the last major revision to IP as it affected the IP layer on hosts (essentially the kernel as regards Unix machines) was sub-netting (RFC 950, 1985), and the new requirements were consolidated in RFC 1122 (1989). For some (but generally not stub) routers, the introduction of CIDR (RFC 1518, 1993) was another major change. RFC 1123 (1989) was the last major re-definition of the requirements for existing applications. Routers have to change faster than hosts, and the last list of requirements for routers was in RFC 1812, though updated by RFC 2644.

   In computing terms, this is relatively stable, since the changes were engineered to be upwards-compatible, as far as possible. The change from IPv4 to IPv6 will be far more fundamental.

2. We have now seen much of the TCP/IP world, so we can review the layering structure we described in the notes to Figures 1.2/1.3.
   
   $$\begin{array}{lccccccc} \hbox to 0in{\rm Level\hss}&\hbox{\r... ...} \cr &&&&\nwarrow&\uparrow \cr 3&&&&&\hbox{\rm IP} \end{array}$$
   
   
   We note that there are now several different level-6 encodings. XDR was defined to solve the comparatively simple problem of transport of integers etc. across a binary protocol (RPC). MIME was invented to transfer structured (e.g. multi-part messages) binary data across a fundamentally 7-bit ASCII connection (SMTP). ASN.1 deals with more complex trees than either.
3. This also lets us contrast UDP with TCP. Most ``long-distance'' applications use TCP, with its sophisticated (some would say complicated) mechanisms for adjusting to the available bandwidth, and recovering from dropped packets. ``Local'' applications such as NFS, other RPC-based systems such as Sun's NIS⁶³, TFTP etc. use UDP. The DNS, which normally transfers a small amount of data but which can require to transfer much more, and NFS are now capable of using either, depending on the situation.

   There was an attempt to define a protocol⁶⁴ with the basic features of UDP but with reliability, however it never went beyond ``experimental'' status.