Examining network packets
Information about the RTL8139
needed for understanding our
‘watch235.c’ pseudo driver
Harney-235
• The Kudlick Classroom’s workstations are
equipped with network interface adapters
designed by RealTek (model 8139)
• The Linux network device-driver for these
NICs is configured on our workstations to
use 32KB of memory as a receive buffer
• Each device interrupt identification number
is calculated by adding 0x20 to the IRQ
RTL8139 registers
• The RTL8139 has a number of hardware
registers which control its operations and
provide information about its current state
• The Linux driver programs these registers
• Our driver will need to read three of them:
– Interrupt Status register
– Receive Buffer Start Address register
– Current Address of Packet Read register
The Receive Buffer
32KB RAM
packet
data
packet-header
packet-checksum
RBSTART
CAPR
ISR
CBR
32-bits
16-bits
16-bits
16-bits
Rx Buffer Operation
• When a new packet is received, the 8139
stores it into the Receive Buffer area, at
the offset specified in the CBA register
(Current Buffer Address), automatically
advances the CBA value to the offset of
the end of the packet, and generates an
interrupt (with ISR equal to 0x0001)
Interrupt Service Routine
• The CAPR register points to an offset that
is 16-bytes ahead of the newly received
packet’s packet-header
• The packet-header is always aligned on a
32-bit address-boundary
• The packet-header consists of two fields:
– The packet status (16-bits)
– The packet length (16-bits)
Packet Length
• The packet-length field contains a count of
the number of bytes of packet-data, plus 4
(for the length of the packet’s checksum)
• The maximum length of a network packet
is set by an ethernet standard: 1514 bytes
(not including the 4-byte CRC checksum)
• The minimum length of a network packet
is normally required to be 60 bytes (+CRC)
Packet Status
M
A
R
P
A
M
B
A
R
I
S
E
R
U
N
T
Legend:
ROK = Receive OK
FAE = Frame Alignment Error
CRC = Checksum Error
LONG = Long Packet ( > 4K bytes)
RUNT = Data + CRC < 64 bytes
ISE = Invalid Symbol Error
BAR = Broadcast Address Received
PAM = Physical Addfess Matched
MAR = Multicast Address Received
L
O
N
G
C
R
C
F
A
E
R
O
K
Demo: ‘trywatch.cpp’
• After you download and then compile our
‘watch235.c’ driver module, you can try
running this demo-program to see a dump
(in hex and ascii formats) of a packet that
is received in our classroom (Harney-235)
In-Class exercise
• Modify the ‘trywatch.cpp’ program so that it
continuously ‘dumps’ received packets,
until the user terminates the endless loop
by typing <CONTROL>-C.
• For a graceful termination, you should use
a signal-handler to break out of your loop,
so that you can close the pseudo-file and
remove the ‘watch235’ driver module