Who s listening Some experiments with an echo service on our anchor cluster s local network of 82573L nic s 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 Memory layout define KMEM SIZE 4 12 12KB kmem 16KB kernel memory allocation 4KB rxring txring Each of the 8 packet buffers has size 1536 bytes 0x600 Each of descriptor table occupies 8 16 bytes 128 bytes Sharing of packet buffers Rx descriptors Tx descriptors 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 Packet processing source becomes destination destn address then our station becomes source mac 6 source address data data data data data data TYPE LENGTH 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
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