Whos listening

1
Whos listening?

• Some experiments with an echo service on our
  anchor-clusters local network of 82573L nics

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 (4ltlt12) // 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
Tx-descriptors
Rx-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 (1ltlt7) ) // 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( cp0, cp6, 6
) // source becomes destination memcpy( cp6,
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
mac6
destn-address
source-address
TYPE/ LENGTH
-- data --
-- data --
-- data
-- data --
-- data --
-- data --
7
In-class experiments

• Lets compile and install our nicecho.c module
  on one (or more) anchor stations
• Then lets transmit a broadcast packet from a
  different anchor station, while we simultaneously
  listen for any replies
• Then lets see what happens if we omit our
  processing of the station-addresses