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with various data-link layer, network layer, routing and transport layer networking protocols. It has been specifically developed for undergraduate teaching."> undergraduate, teaching"> TEXT ="black" LINK="blue" VLINK="purple">

cnet's Physical Layer

The Physical Layer (either the internal default version or one provided with the -R option) has the responsibility of delivering data frames between nodes. Frames are delivered along links, numbered within each node from 1 to the number of links. Link 0 is provided to copy a frame immediately from a node's output to input. In general, the Physical Layer will randomly corrupt and drop data frames on all links other than 0.

When your protocols wish to transmit a data frame along a link, they write that frame to the Physical Layer. On calling the CNET_write_physical function, you indicate the length of the frame to be written and on return CNET_write_physical indicates how many bytes were accepted. A typical sequence for a 2 node network is: 


char myframe[ MAX_MESSAGE_SIZE + MY_OVERHEAD ];
int  framelength;

 ...   /* prepare frame contents for transmission */
framelength = ... ;
result = CNET_write_physical(1, myframe, &framelength);

When cnet informs the destination node that a frame has arrived, the handler for EV_PHYSICALREADY should read that frame. On return from a successful call to CNET_read_physical, your protocol is informed on which link the frame arrived and how long it was. Of course, in a simple 2 node network all frames will arrive on link number 1. 


char myframe[8*K];
int  link, length;

length = sizeof(myframe);
result = CNET_read_physical(&link, myframe, &length);
 ...   /* process frame contents */

Two additional Physical Layer functions are provided to assist in the debugging of multi-layered protocols. CNET_write_physical_reliable is identical to CNET_write_physical except that frames sent using it will not be subject to frame corruption or loss. It can be considered as a ``perfect'' Data Link Layer if you just want to implement higher-layered protocols. The function CNET_write_direct also bypasses all Physical Layer errors and instructs a message to be sent directly to the node whose address is specified as a parameter. It thus provides perfect a Data Link Layer and Network Layer.

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Physical Layer functions

int CNET_write_physical(int link, char *frame, int *len);

Passes a number of bytes, pointed to by frame ``down to'' the Physical Layer which will attempt to deliver them on the indicated link (wire). Each node has a fixed number of links, the first available link is number 1, the second is number 2, and so on. As a special case, a node may reliably transmit a frame to itself by requesting the LOOPBACK(=0) link. On invocation, len must point to an integer indicating the number of bytes to be taken from frame. On return, len will point to an integer now indicating the number of bytes accepted by the Physical Layer.

Possible errors: ER_BADARG, ER_BADLINK, ER_BADSIZE, ER_LINKDOWN, ER_NOTREADY, ER_TOOBUSY.

int CNET_write_physical_reliable(int link, char *frame, int *len);

Identical to CNET_write_physical though the transmission is guaranteed to be error free (providing a reliable data-link layer).

Possible errors: ER_BADARG, ER_BADLINK, ER_BADSIZE, ER_LINKDOWN, ER_NOTREADY, ER_TOOBUSY.

int CNET_write_direct(CnetAddr destaddr, char *msg, int *len);

Similar to CNET_write_physical_reliable but the network address of the required destination node may be specified (providing a reliable network/routing layer for asynchronous message passing). Messages transmitted using CNET_write_direct are considered to be transmitted on, and arrive on, link number 1. The special destination address BROADCAST may be used to transmit a message to all nodes except the sender. Regardless of the number of hops, propagation delays, and bandwidth, all frames sent via CNET_write_direct reach their destination in just 1msec.

Possible errors: ER_BADARG, ER_BADLINK, ER_BADSIZE, ER_LINKDOWN, ER_NOTREADY, ER_TOOBUSY.

int CNET_read_physical(int *link, char *frame, int *len);

Accepts the specified maximum number of bytes from the Physical Layer, placing them in the address pointed to by frame. On invocation, len must point to an integer indicating the maximum number of bytes that may be copied into frame. On return, len will point to an integer now indicating the number of bytes taken from the Physical Layer and link will point to an integer indicating on which link they were received.

Possible errors: ER_BADARG, ER_BADSIZE, ER_CORRUPTDATA, ER_NOTREADY.

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cnet was written and is maintained by Chris McDonald (chris@cs.uwa.edu.au)