Protocol Debug

1
Protocol Debug

• Review Ethernet
• Debug Techniques

2
Ethernet
3
Overview

• History
• Developed by Xerox PARC in mid-1970s
• Roots in Aloha packet-radio network
• Standardized by Xerox, DEC, and Intel in 1978
• Similar to IEEE 802.3 standard
• Manchester encoding, synchronous transmission

4
Frame Format

• Addresses
• Unique, 48-bit unicast address assigned to each
  adaptor
• Example 802be4b12
• Broadcast all 1s
• Multicast first bit is 1

5
Addressing

• Adaptor receives all frames it accepts (passes
  to host)
• Frames addressed to its own unicast address
• Frames addressed to the broadcast address
• Frames addressed to any multicast address it has
  been programmed to accept
• All frames when in promiscuous mode

6
Transmitter Algorithm

• If line is idle
• Send immediately
• Upper bound message size of 1500 bytes
• Must wait 51?s between back-to-back frames
• If line is busy
• Wait until idle and transmit immediately
• Called 1-persistent (special case of
  p-persistent)
• Detect collision when transmitter differs from
  receiver

7
Collision

• jam for 512 bits, then stop transmitting frame
• minimum frame is 64 bytes (header 46 bytes of
  data)
• delay and try again
• 1st time uniformly distributed between 0 and
  51.2?s
• 2nd time uniformly distributed between 0 and
  102.4?s
• 3rd time uniformly distributed between 0 and
  204.8?s
• give up after several tries (usually 16)
• exponential backoff

8
Collisions
Jam
9
Minimum Packet Size

• Packet must fill whole pipe in order to block
  other hosts from starting transmission
• The packet must continue until a collision is
  detected from a transmission starting just before
  the original frame arrived
• The minimum packet size is twice the number of
  bits needed to fill the pipe

10
Example

• 10 base T- 2500 meter length, 10Mbps
• At Speed of Light 2108m/s

11
Experiences

• Observe in Practice
• 10-200 hosts (not 1024)
• Length shorter than 2500m (RTT closer to 5? than
  51?)
• Packet length is bimodal
• High-level flow control and host performance
  limit load
• Recommendations
• Do not overload (30 utilization is about max)
• Implement controllers correctly
• Use large packets
• Get the rest of the system right (broadcast,
  retransmission)

12
Protocol Testing

• Hardware Testing
• Sniffers
• Debug Statements

13
Hardware Testing

• Loopback testing
• Modems normally have this option
• RS232 cables can send output back to the input
  pins
• Analog Test Equipment
• Network analyzers will display and capture in a
  non-intrusive way

14
Sniffers

• Ethernet card is placed in promiscuous mode
• Packets are decoded at each level
• Normally the raw data is displayed.
• Example netmon for NT, tcpdump for linux
• Can isolate attacks

15
Debugging your code

• Posted Code in physical lab description gives you
  a familiar format

00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e
0f   ........ ........10 11 12 13 14 15 16 17 
18 19 1a 1b 1c 1d 1e 1f   ........ ........20 21
22 23 24 25 26 27  28 29 2a 2b 2c 2d 2e 2f   
!"' (),-./30 31 32 33 34 35 36 37  38 39
3a 3b 3c 3d 3e 3f   01234567 89ltgt?
16
Dont delete your debug code!
define DEBUG_PHYS 0x01 define
DEBUG_ETH_BITSTUFFED 0x02 define DEBUG_ETH_SANE
0x04 define DEBUG_ETH_ADDRS 0x08 if (blabby
DEBUG_ETH_SANE) print_chars(ethernet, buff,
length)
17
Assert

• Protect yourself from insanity later in life by
  visiting a psychiatrist in kindergarten
• DESCRIPTION
• assert() prints an error message to
  standard output and terminates the program by
  calling abort() if expression is false (i.e.,
  compares equal to zero). This only happens when
  the macro NDEBUG is undefined.

18
Assert Example

• You know that the pop from the physical routine
  should be less than 90 bytes.
• pop ( char buf, int len)
• assert(len lt 90)
• If len is ever greater than 90, an error will be
  printed out an the code will exit.

19
Printf(error d\n,__LINENO__)
20
Test for boundary conditions

• Check for 0xff ff ff ff ff ff addresses
• Test for zero byte and max length messages
• Test Harness above and below layer
• Smoke Test

21
Debug Techniques

• Real Men use gdb
• g -g foo.cc o foo (turn on debug flag)
• gdb ./foo
• run arguments (run 460one 1234 s)
• When is crashes
• where will give you a backtrace of what was
  called
• break lets you set breakpoints
• s single steps
• print prints out variables
• There are also guis (xxgdb, kdevelop)

22
What about segmentation faults?

• The segmentation fault can occur in something
  like libc, but that doesnt mean you are absolved
  from finding it.
• Most problems come when you are writing beyond
  the end of the buffer or have an uninitialized
  pointer
• Look at hosed pointer and see if you can spot a
  pattern (like the data you were receiving)

23
An ounce of prevention

• The best way to avoid these problems is to fix
  every anomaly
• Even if it doesnt seem critical it will bite you
  later
• Debug each layer thoroughly