Q1. Checksum: 10000002 11100110
b. Checksum: 10000001 11100111
Q1 a) To checksum two bytes of data, the two bytes can be concatenated and treated as a 16-bit word. The checksum can be calculated by adding two bytes using one's complement representation. The checksum is the one's complement of the sum. for example:
Byte 1:
00110100
Byte 2:
01101001
Linking:
00110100 01101001
total:
01111101 00011001
One's complement:
10000002 11100110
Checksum: 10000002 11100110
b) If the least significant bit of byte1 is corrupted to 1 and the least significant bit of byte2 is corrupted to 0, the resulting data will be:
Byte 1:
00110101
Byte 2:
01101000
Linking:
00110101 01101000
total:
01111110 00011000
One's complement:
10000001 11100111
Checksum: 10000001 11100111
c) No, if the Internet checksum matches the value contained in the checksum field, the recipient cannot be completely certain that no bit errors have occurred. Internet checksums are used to detect errors in data during transmission, but are not a foolproof method. There may still be errors in the data that the internet checksum failed to detect.
Q2)
a) If her ACK was dropped on the network and never reached the sender with a stop-and-wait protocol, the sender should time out and resend the packet.
b) If the sender sends his 100 packets, 80 packets successfully reach the destination, and 20 packets are dropped in the network, the receiver receives Send his 80 his ACKs (acknowledgments). The receiver does not send her NACK (negative ACK) because all packets were received successfully.
c) The UDP header length field is required to specify the length of the UDP datagram, including the UDP header and payload. The UDP header length is fixed at 8 bytes, but the payload length can vary. The length field is necessary so that the receiver can determine the end of her UDP datagram and process the payload correctly.
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