1. Who’s listening?
Some experiments with an ‘echo’ service on our anchor-cluster’s local network of 82573L nic’s

2. ‘nicecho.c’ Module uses a set of eight packet-buffers
Same buffer is used for both RX and TX
The NIC operates in “promiscuous” mode
Interrupt-hander ‘reflects’ each received packet back to the station it came from
Pseudo-file shows both descriptor-queues

3. Memory-layout
#define KMEM_SIZE (4<<12) // 16KB kernel memory allocation
12KB
4KB
rxring
txring
kmem
Each of the 8 packet-buffers has size 1536 bytes (=0x600)
Each of descriptor-table occupies 8*(16 bytes) =128 bytes

4. Sharing of packet-buffers
Rx-descriptors
Tx-descriptors

5. Interrupt-processing
irqreturn_t my_isr(int irq, void *dev_id ) {
int intr_cause = ioread32( io + E1000_ICR );
if ( intr_cause == 0 ) return IRQ_NONE;
if ( intr_cause & (1<<7) ) // receiver timeout: a new packet arrived
int index = ioread32( io + E1000_TDT );
unsigned long where = txring[ index ].base_address;
unsigned char *cp = phys_to_virt( where );
memcpy( cp+0, cp+6, 6 ); // source becomes destination
memcpy( cp+6, mac, 6 ); // our station becomes source
txring[ index ].desc_status = 0;
txring[ index ].packet_length = rxring[ index ].packet_length;
index = (1 + index) % 8;’ // ring-buffer index advances
iowrite32( io + E1000_TDT ); // start sending packet back }
iowrite32( intr_cause, io + E1000_ICR ); // clear the interrupts
return IRQ_HANDLED;

6. Packet-processing …then our station becomes source
source becomes destination…
mac[6]
destn-address
source-address
TYPE/
LENGTH
-- data --
-- data --
-- data –
-- data --
-- data --
-- data --

7. In-class experiments
Let’s compile and install our ‘nicecho.c’ module on one (or more) anchor stations
Then let’s transmit a ‘broadcast’ packet from a different anchor station, while we simultaneously listen for any replies
Then let’s see what happens if we ‘omit’ our processing of the station-addresses