Outline

• Error Detection
  • Theory
  • CRC
  • Internet checksum
• Network Adapters
  • Architecture
  • Device Drivers

Error Detection

• bit errors
  • random change in value of a bit
  • causes:
    • thermal noise
    • timing
    • marginal (failing) equipment/connectors
    • temperature or power levels
  • characterized by Bit Error Rate (BER)
• burst errors:
  • setting a sequence of bits to one or zero, or flipping a sequence of bits
  • causes:
    • electrical interference
    • sudden service interruption ("backhoe")
  • characterized by burst length, burst rate

Error detection

• strategy: include additional information with what you send so receiver can check integrity
• "code distance": how many bits in a packet must I flip in order to get an erroneous valid packet
• effectiveness: how many bits errors/packet can be flipped without escaping detection
• if an erroneous packet is detected, can try to reconstruct original data (error correction) or discard packet (retransmission)
• parity bit detects all odd bit errors: 1, 3, ...
• internet checksum
• CRC

Error correction

• strategy: include additional information with what you send so if there are any errors, receiver can reconstruct data
• error correction often implies error detection
• XOR: send N packets followed by one packet that has the XOR of all preceding packets. If only one of the N packets is corrupted, we can reconstruct it from the other N-1 packets, plus the XOR
• triple-redundancy: send each packet 3 times, use majority voting to reconstruct original packet
• CRC
• used when retransmission is impractical (high latency links) or impossible (storage systems)

Checksum

• 16-bit Internet checksum used:
  • over IP header,
  • over TCP (mandatory) and UDP (optional) header, pseudo-header, data
  • in other Internet protocols, e.g. ICMP
• RFC 1071 (http://www.cis.ohio-state.edu/htbin/rfc/rfc1071.html).
• 1's complement sum of all pairs of bytes (padded with a zero byte if needed)
• on receipt, the sum must be all 1s
• detects all single-bit errors
• detects most burst errors
• does not detect 16-bit reorderings

2's complement arithmetic

• "Normal" binary addition is 2's complement arithmetic: the negation of a number is the complement of the number, plus one
• example: negation of 0011 is 1101
• 2's complement has exactly one representation for the value zero

1's complement arithmetic

• In 1's complement arithmetic, the negation of a number is the complement of the number.
• example: negation of 0011 is 1100
• 2's complement has exactly one representation for the value zero
• 1's complement has two representations for the value zero: all zeros and all ones
• 1's complement 16-bit sum is commutative, associative, and byte-order independent.

Implementing 1's complement addition

• computer addition is generally 2's complement
• implementation: when the addition of two numbers generates a carry (in 2's complement arithmetic), simply add the carry back in

  2's	(10's)	1's	(10's)
  1100 +	-4 +	1100 +	-3 +
  0101 =	5 =	0101 =	5 =
  1 0001	1(17)	0010	2

• Using a 32-bit value to compute the 16-bit checksum, the carries can be accumulated, then added in at the end.

In-class exercise

• compute the 16-bit checksum of the following 8 bytes of data (given in hex):

  12 34 56 78 8a bc de f0
  

Network devices

• NIC (network interface card)
• Physical Layer
• move bytes from the host to the network and from the network to the host
• architecture: controller, bus interface, network interface

Bus Interface

• move bytes from the host to the adapter and from the adapter to the host
• very simple for programmed I/O
• more complex for DMA
• complexity:
  • arbitration
  • bus access

Network Interface

• move bits from the adapter to the network and from the network to the adapter
• convert bits to bytes
• synchronize with sender or receiver
• complexity:
  • media access control (MAC) protocol
  • synchronization with sender/receiver
  • high speed device
  • buffering

Network Device Controller

• very simple computer
• often programmable
• interrupt the host
• interpret commands in the control register
• set the status bits
• may do other functions such as computing the CRC, managing internal buffers

Device Drivers

• the operating system component that communicates with a device
• does input, output, and management
• lower half is executed at interrupt time:
  • does the actual I/O and management
  • signals the upper half
• upper half is executed at system call time:
  • requests I/O or management functions from lower half
  • usually does resource management
  • may cause device to interrupt, so lower half is executed
• communication often by shared state, so synchronization is essential