# Q3(b)(i) · Why rdt3.0 needs a timer

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2023-jan-ai-reexam/q3bi

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2023-jan-ai-reexam--q3bi/

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

### Q3(b)(i). If the round-trip delay is constant and known to the sender, is a timer still necessary in rdt3.0 when packets may be lost? Explain. [4 marks]

### Answer

**Yes.** A known RTT tells the sender when a response should arrive, but a lost DATA or ACK produces no arrival event. A timer is required to detect that absence and trigger retransmission. It can be set using the known RTT plus appropriate transmission/processing allowance. Example: DATA0 is lost; without a timer the sender could wait forever forACK0. A timeout repeats DATA0, while the sequence bit prevents duplicate delivery if only the ACK was lost.