# Q3(b)(i) · UDP checksum limitations

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2022-ai-final/q3bi

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2022-ai-final--q3bi/

Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.

### Q3(b)(i). A UDP receiver computes the Internet checksum and it matches the checksum carried in the segment. Can it be absolutely certain that no bit errors occurred? Explain. [5 marks]

### Answer

**No.** An Internet checksum compresses a long message into16 bits, so different messages can have the same sum. Compensating changes can leave the one’s-complement sum unchanged. For example, word pair0001,0002 and0000,0003 (shown in hexadecimal) both sum to0003 and have the same complement checksum. A matching checksum means “no error detected”; it does not guarantee bit-for-bit identity. UDP does not automatically repair errors or retransmit lost data.