Question

In a UDP packet, if its payload has two 32 bit intigers: E666

and D555, then its checksum should be:
A. 4443
B. 4444
C. 5454
D. 616F

Answer 1

To calculate the checksum for a UDP packet with payload integers E666 and D555, each 16-bit word is added with end-around carry, and the sum is one's complemented. The checksum for the given UDP packet payload is 4433, corresponding to option A.

Step-by-step explanation:

The question asks to calculate the checksum for a UDP packet given two 32-bit integers in its payload. When calculating the checksum, each word (16 bits) is added and any overflow that occurs is wrapped around (also known as end-around carry). This process continues for all words in the payload. Once all words are added, the final sum is then one's complemented (meaning each bit is inverted) to obtain the checksum.

In hexadecimal, the payload integers are given as E666 and D555. To find the checksum, we break these into 16-bit words and add them:

• E666 (high word of first integer)
• D555 (low word of first integer)

The hexadecimal addition is:

• E666
  + D555
  ————
  = 1BBCB

Since there is an overflow (1), we perform an end-around carry:

• BCB
  + 1
  ———
  = BCC

The result of adding the two integers is BCC. Now we one's complement the result to find the checksum:

• Checksum = ~BCC (one's complement of BCC)
  = 4433

Therefore, the checksum for the given UDP packet payload is 4433, which corresponds to option A.