[COFF] Fwd: [ih] NCP and TCP implementations

Given the recent discussion of pipes and networking ... I'm passing this along for those that might not have seen it. ---------- Forwarded message --------- From: Jack Haverty via Internet-history Date: Tue, Mar 10, 2020 at 1:30 PM Subject: Re: [ih] NCP and TCP implementations To: *Internet-History* The first TCP implementation for Unix was done in PDP-11 assembly language, running on a PDP-11/40 (with way too little memory or address space). It was built using code fragments excerpted from the LSI-11 TCP implementation provided by Jim Mathis, which ran under SRI's home-built OS. Jim's TCP was all written in PDP-11 assembler. The code was cross-compiled (assembled) on a PDP-10 Tenex system, and downloaded over a TTY line to the PDP-11/40. That was much easier and faster than doing all the implementation work on the PDP-11. The code architecture involved putting TCP itself at the user level, communicating with its "customers" using Unix InterProcess Communications facilities (Rand "Ports"). It would have been preferable to implement TCP within the Unix kernel, but there was simply not enough room due to the limited address space available on the 11/40 model. Later implementations of TCP, on larger machines with twice the address space, were done in the kernel. In addition to the Berkeley BSD work, I remember Gurwitz, Wingfield, Nemeth, and others working on TCP implementation for the PDP-11/70 and Vax. The initial Unix TCP implementation was for TCP version 2 (2.5 IIRC), as was Jim's LSI-11 code. This 2.5 implementation was one of the players in the first "TCP Bakeoff" organized by Jon Postel and carried out on a weekend at ISI before the quarterly Internet meeting. The PDP-11/40 TCP was modified extensively over the next year or so as TCP advanced through 2.5, 2.5+, 3, and eventually stabilized at TCP4 (which it seems we still have today, 40+ years later!) The Unix TCP implementation required a small addition to the Unix kernel code, to add the "await" and "capac" system calls. Those calls were necessary to enable the implementation of user-level code where the traditional Unix "pipeline" model of programming (input->process->process...->output) was inadequate for use in multi-computer programming (such as FTP, Telnet, etc., - anywhere where more than one computer was involved). The code to add those new system calls was written in C, as was almost all of the Unix OS itself. The new system calls added the functionality of "non-blocking I/O" which did not previously exist. It involved very few lines of code, since there wasn't room for very many more instructions, and even so it required finding more space by shortening many of the kernel error messages to save a few bytes here and there. Randy Rettberg and I did that work, struggling to understand how Unix kernel internals worked, since neither of us had ever worked with Unix before even as a user. We did not try to "get it right" by making significant changes to the basic Unix architecture. That came later with the Berkeley and Gurwitz efforts. The PDP-11/40 was simply too constrained to support such changes, and our mission was to get TCP support on the machine, rather than develop the OS. I think I speak authoritatively here, since I wrote and debugged that first Unix TCP code. I still have an old, yellowing listing of that first Unix TCP. FWIW, if there's interest in why certain languages were chosen, there's a very simple explanation of why the Unix implementation was done in assembler rather than C, the native language of Unix. First, Jim Mathis' code was in assembler, so it was easy to extract large chunks and paste them into the Unix assembler implementation. Second, and probably most important, was that I was very accustomed to writing assembler code and working at the processor instruction level. But I barely knew C existed, and was certainly not proficient in it, and we needed the TCP working fast for use in other projects. The choice was very pragmatic, not based at all on technical issues of languages or superiority of any architecture. /Jack Haverty On 3/9/20 11:14 PM, vinton cerf via Internet-history wrote: -- Internet-history mailing list Internet-history(a)elists.isoc.org https://elists.isoc.org/mailman/listinfo/internet-history