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.
Other Utilities

Eduardo also provides the following useful DECnet utilities:

  • sethost provides terminal access to OpenVMS machines, similar to TELNET.

  • ctermd is a daemon that provides the opposite service, allowing OpenVMS users to SET HOST (or TELNET if you prefer) to a Linux machine.

  • dnmirror and dnping are test utilities for checking that the software is installed correctly and verifying connections to particular OpenVMS nodes.

All the above utilities and the X servers depend on libdnet which is available from our web site (see Resources).

A Tour of the Kernel Sources

For the kernel hackers, here is a quick spin around the relevant source files. This section applies only to the newer kernel patches (i.e., for the 2.1.xx series and up), as those are the ones we expect you'll find most interesting and useful.

Figure 1. DECnet Software Layer Diagram

A diagram showing the overall layout of the Linux DECnet layer is shown in Figure 1. Where we describe the DECnet protocol, we go into just enough detail to give an idea of the main features of each part of the kernel code. If you want to know more about the way the kernel code works, you will need to read a copy of the DECnet specifications (see Resources) and then look at the source directly.

The main source file is af_decnet.c. This file contains the socket layer interface and parts of the DECnet NSP and session control layer code. Since the DECnet layering model does not map exactly onto the socket code, session control is provided partly by the kernel and partly by a user space library called libdnet.so.

dn_raw.c contains the code which implements the raw socket layer. It was one of the first things written, since it is very useful when debugging to see what is going on “under the hood”. It is also a good example of how to write the simplest socket layer interface possible. The file is compiled only when the raw sockets option is configured.

Figure 2. State Diagram of the NSP Layer

The rest of the NSP layer is divided into two parts: one for dealing with outbound packets, dn_nsp_out.c, and one for incoming packets, dn_nsp_in.c. The state table for the NSP layer is shown in Figure 2. We won't say much about the diagram here, but it should be a useful aid when used in conjunction with the kernel code.

The Routing layer is rather problematical. It has been divided into several files, due to the fact that the Routing layer actually does much more than just routing. dn_route.c is the main file which deals with incoming and outgoing packets, and dn_dev.c provides support for device-specific functions.

dn_neigh.c has a split personality. When a node is running as an endnode, it provides the On-Ethernet Cache described in the DECnet specifications; for routers, it provides the adjacency database. Since they are so similar, the decision was made to merge the two functions in order to keep the code small.

The actual routing functions (compiled only when the node is configured as a router) are in dn_fib.c. The code in this file is very experimental at this time, as decisions are still being made regarding how much of the routing should be done in user space and how much in kernel space.

Main Kernel Code Paths

One of the more obscure and important parts of the code is the main path for outgoing data packets. The DECnet layer uses the protocol-independent destination cache written by Alexey Kuznetsov, and neighbor table code written by Alexey Kuznetsov and Pedro Roque. These were originally designed to do some common processing required by the IPv4 and IPv6 network protocols, with the intention that other protocols would begin to use them at a later date.

What exactly do these two bits of code do? We will start by describing the neighbor table. The idea behind this is simply to keep a list of known nodes which are directly connected, along with certain information used by the protocol in question to communicate with them. In the case of TCP/IP, this means the ARP subsystem; for DECnet, it is used to hold one of two things. For endnodes, it holds the list of known nodes on the directly connected networks with which communication can be established, known as the On-Ethernet Cache. For routing nodes, it holds what the specifications describe as the adjacency database. In both cases, the function is the same but the actual method of operation is slightly different.

In the endnode case, hello messages are sent by routers every ten seconds to all endnodes that are directly connected. They are used by the endnode to create entries in the On-Ethernet Cache. Should a hello message not be received for a certain length of time, normally one minute, the entry is removed from the list. A default router is a directly connected node to which packets should be sent if the endnode is not connected directly to the destination. The default router is determined by information in the received hello messages.

For routing nodes, hello messages are received from both endnodes and other routers and are used to update the adjacency table. In this case, the entries are removed if no hello messages are received for a length of time—a specified multiple of a time length noted in the hello messages. Currently, the neighbor table does not support different timeouts for each different neighbor. This problem is being worked on and may be solved by the time you read this.

One other piece of information held in the neighbor table is the format of header to be used by the routing layer in transmitting NSP data. There are two formats, one for use over broadcast links (long format) including Ethernet, and one for use over point-to-point links (short format). This is done by setting a pointer to a function to the correct routine dn_long_outout or dn_short_output when the table entry is created.

The destination cache is based on principles similar to the neighbor table. However, the object is to hold information required for each destination. When a packet is to be sent to a certain destination, it is looked up in the destination cache to see if it exists. If so, then that entry is used; if not, a routing algorithm must be called to discover the correct destination.

The routing algorithm also depends upon the routing or non-routing type of the node. The algorithm for routers has not been properly implemented yet, but will reside in the file dn_fib.c when that time comes. For endnodes, the algorithm is simply to send directly to any node in the On-Ethernet Cache, to send to the default router if it is not in the cache, or to send directly if there is no default router.

Again, a function pointer is available in each destination entry for a routine that will add destination-specific information to outgoing packets, then call the output routine of the neighbor.

That about wraps up the main features of the kernel code. There is, of course, a lot more to it than what is mentioned here, but we hope our overview will be useful if you're planning to add features or help with debugging. If you have specific questions, we'd be happy to try to answer them; however, please read the documentation first and also remember that we may not always be able to send an answer right away.

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