DECnet Network Protocol

This article contains information on how to use and configure available DECnet software as well as information on how the kernel code works.

DECnet was designed by Digital as a way to interconnect their range of products. In its Phase IV implementation, released in 1983, it can support 63 areas of 1023 nodes each. The specifications for DECnet Phase IV are freely available (see Resources), which has allowed others to provide DECnet connectivity in products such as Sun's Sunlink DNI and Linux.

In networking terms, DECnet is an old standard. Its limited address space is far less than that of TCP/IP and it does not have the advanced features of more modern networking standards. DECnet is still widely used in legacy systems, and the intention of the Linux DECnet project was to allow integration of those systems with Linux-based solutions.

The Linux DECnet project intends to support only Phase IV DECnet, since previous versions in current use are very limited in number these days. LAT, another network protocol designed by Digital, will not be supported because it is covered by patent protection and its specifications are not freely available. In this article, we will use the term “DECnet” to mean the DECnet Phase IV family of protocols.

The DECnet Family of Protocols

DECnet can be carried over a variety of different data link layers. In the beginning, the Linux kernel DECnet layer will support only the Ethernet link layer; support will be added later for other link layers such as PPP, DDCMP and X.25. The PPP link layer is described in RFC1762 and the others are described in the DECnet documents (see Resources).

Like many network protocols, DECnet can be viewed as consisting of a number of software layers. More details are included in the section called “A Tour of the Kernel Sources”.

At the top of the stack is the application layer, which includes all the programs used on a day-to-day basis. These programs use the system libraries and system calls to create connections to other nodes. The kernel socket layer interface and the system libraries encompass what the DECnet standard refers to as the session control layer. It performs largely the same function as the library and system calls for TCP/IP. Further down is the network services protocol (NSP), fairly close to TCP in function. Below these is Routing, which does more than routing; it is a kind of IP and ARP rolled into one. At the bottom of the heap is the actual device over which the data is transmitted.

Each machine, called a node in a DECnet network, is identified by an address consisting of a 6-bit area number and a 10-bit node number. These two numbers are written separated by a dot, so 1.2 is a computer in area 1 with a node number of 2. Unlike TCP/IP, the address refers to the computer, not the interface through which communication is to take place.

Kernel Configuration

Two different sets of patches are available to add DECnet support to the kernel. The currently available code is based upon a patch written by Eduardo Serrat for the 2.0.xx version kernels to function as an endnode. In parallel with this, one of the authors, Steve Whitehouse, was also writing a DECnet layer with an emphasis on creating a router implementation.

The result is you can now get the original patch for 2.0.xx kernels written by Eduardo Serrat (a version of the same code ported to the 2.1.xx kernel series) and also another patch which has modifications by Steve to make use of the newer support functions in the 2.1.xx kernel series. This last patch will be distributed as an add-on for the 2.2.xx kernel series and later integrated into the next development series.

Most of what we say here applies equally to all versions of the kernel patches. We will point out the differences as we go.

First, you need to retrieve the correct patch for your kernel. In general, the best way to do this is to get a copy of the most up-to-date kernel in whichever kernel series you intend to use. You can then download and apply the latest patch to the kernel source as described below. I also strongly suggest that you study the release notes for the kernel version you have, since the configuration procedure may change.

To apply the Linux DECnet patch, unpack the kernel source into /usr/src/linux as normal. Then obtain the correct patch for this kernel version and uncompress it in the directory above the top-level Linux source directory, /usr/src/ in this case. Then type:

$ patch -p0 < patch-file

patch-file is the name of the patch you want to apply. Next you need to compile a kernel in the usual way, being sure to say Y or N to the question about including DECnet support. Depending on which version of the patch you have, some of the options outlined below may be available.

The two main types of DECnet nodes are endnodes and routers; the latter is subdivided into Level 1 and Level 2 varieties. At the time of writing, only endnode support is available.

When DECnet router support is available, you will need to enable the option at compile time. Also, at module load or boot time, you will have to turn it on—a kernel with router support compiled in will be able to function as both an endnode and a router.

The DECnet raw sockets option allows the reading and writing of DECnet packets at a lower level than users normally need. It is very useful for debugging and monitoring activity, and might be required by future user-level routing daemons. The main advantage of using this rather than a PF_PACKET socket is allowing a program to take advantage of the kernels filtering of invalid DECnet packets.

In order to use the DECnet kernel layer, you must also tell the kernel which DECnet address to use. This is the only point at which the instructions are dependent upon the kernel version being used. For 2.0.xx version kernels, you need the startnet program, which would normally be run in the boot scripts for your system. For the 2.1.xx version kernels and upward, the DECnet address can be set either on the command line with an option decnet=1,2 or when the module is loaded. (That is a comma, not a dot, between the 1 and 2 in the previous sentence.)