Topology files, nodes and links

Links are numbered within each node from 0 to the number of physical links that the node has, with 0 representing the LOOPBACK link. The first ``real'' link is number 1 and every node will have a link number 1.

Perhaps surprisingly, the nodes initially have very little knowledge of the network. Nodes do not know how many other nodes there are, what the other nodes are called, nor the attributes of any nodes or links other than their own. All inter-node communication necessary to learn this information must traverse the Physical Layer.

Topology files

If necessary, the topology file is first preprocessed by the C-preprocessor, enabling us to use #include, #define and conditional ``compilation'' directives if required. C and C++ style comments are also supported.

    /* A simple 2 node network topology */

    compile          = "stopandwait.c"
    messagerate      = 500ms,
    propagationdelay = 700ms,
    probframecorrupt = 3,
    ostype           = "linux"

    host Perth {
	x=100, y=100
	messagerate  = 1000ms,

	link to Sydney
    }

    host Sydney {
	east of Perth

	link to Perth
	    { probframeloss = 2 }
    }

Node attributes and link attributes declared before any nodes are considered global attributes - these will be the defaults unless redefined locally within a node or link definition. Local attributes are declared in a new ``block'', by opening a curly bracket (as in C). In the above topology, the default messagerate (the rate at which the Application Layer will generate a new message for delivery) is 500ms. This becomes the default messagerate for all nodes, but node Perth later declares its own (local) messagerate as 1000ms.

The compile attribute indicates which C source files are to be compiled and executed by cnet. Here, 2 instances the source code in the single file stopandwait.c will be executed by Perth and Sydney. Each node will have its own copy of all variables declared in the file stopandwait.c (globals, static globals, locals and static locals).

Node attributes and node information

Times are stored internally in milliseconds though in the topology file their integral values may be followed by suffixes such as ms and s.

Data sizes are stored internally in bytes though in the topology file their integral values may be followed by suffixes such as bytes, Kbytes, KB, and MB.

Boolean attributes may take on the values true, false, and toggle.

Strings are enclosed within double quotes.

While executing, each node has access to its own CnetNodeinfo structure describing the node's attributes. This structure is best considered read-only as its contents are ``refreshed'' as each node is scheduled for execution.

    typedef struct {
      CnetNodetype  nodetype;       /* Either a NT_HOST or a NT_ROUTER */
      int           nodenumber;     /* Ranging from 0.._NNODES-1 */
      CnetAddr      address;        /* Possibly different to the nodenumber */
      char          *nodename;
      int           nlinks;         /* Ranging from 0(=LOOPBACK) .. nlinks */
      int           minmessagesize; /* min size (in bytes) of msgs generated */
      int           maxmessagesize; /* max size (in bytes) of msgs generated */
      int           messagerate;    /* rate of msg generation (in ms) */
      long          time_in_ms;     /* a monotonically increasing clock */
      struct {
	  long      sec;
	  long      msec;
      }             time_of_day;    /* a reflection of the wall-clock time */
    } CnetNodeinfo;

    CnetNodeinfo  nodeinfo;

Link attributes and link information

Link bandwidths are stored internally in bits-per-second though in the topology file their integral values may be followed by suffixes such as bps, Kbps, and Mbps.

Probabilities specify a uniform distribution, with their value being the log-base-2 of the chance of failure (yes, this is ugly). In the topology above, the global probframecorrupt attribute declares that a frame will be corrupted with probability of 1 in 8 (2 to the power 3) and the link from Sydney to Perth will lose (on average) every fourth frame. A probability of 0 (the default) means that no errors will be introduced.

While executing, each node has access to its own array of read-only CnetLinkinfo structures:

    typedef struct {
	int       linkup;               /* TRUE if link not severed */
	int       bandwidth;            /* in bits per second */
	int       propagationdelay;     /* in ms */
	int       transmitbufsize;      /* in bytes */
	int       costperbyte;          /* in cents(?) */
	int       costperframe;         /* in cents(?) */
    } CnetLinkinfo;

    CnetLinkinfo *linkinfo;    /* linkinfo[0]..linkinfo[nodeinfo.nlinks] */

Compilation Strings

Strings are used to declare the location (filenames) of the source and shared object codes for the Application, ``Central'' and Physical Layers used in each simulation. These strings may be provided on the command line, via the -A, -C, and -P options, or via the compile node attribute in the topology file.

In their simplest form, compilation strings may present just a single C sourcefile name, such as "protocol.c". If necessary, cnet, will compile the file protocol.c into the object file protocol.o and then link this file to form the final shared object protocol.cnet. This final shared object file will then be used to provide the code for each node's relevant layer(s).

In its more complex form, a compilation string may also include compilation switches, a number of sourcefile names, and linker switches. For example, the compilation string

includes an embedded (actually preprocessor) switch which is passed onto the compilation process, two sourcefile names and a linker switch (in this case to link with the mathematics library). Each source file is compiled (if necessary) to create its object file, and all object files are then linked together to form a single shared object. The shared object's name is derived from the first sourcefile found, in this case it will be ftp.cnet. The embedded switches -l and -L are recognized as (assumed to be) linker switches; all other switches are assumed to be preprocessor and compiler switches.

cnet performs the rudimentary actions of make(3), compiling and linking files if the required target does not exist or is out-of-date with respect to sourcefiles.