13 / 13 Congestion Control & Flow Control

The Link Came Back

A phone on a cellular link has 40 unacknowledged segments in flight when a handover freezes the radio for 900 ms. Nothing is lost, only delayed. The sender's RTO is 600 ms and fires during the freeze. Standard Reno, no SACK, no timestamps.

What happens when the link comes back?

A

Exponential backoff doubles the RTO to 1200 ms, so the sender simply waits longer and cwnd is left untouched.

B

The retransmission triggers duplicate ACKs, so the sender enters fast recovery and pays a single halving of cwnd.

C

Nothing is wasted: the cumulative ACK covering the whole window arrives first and cancels the pending retransmissions.

D

The sender already dropped cwnd to one segment and cut ssthresh at the timeout, retransmitted the oldest segment, and then goes on to resend the rest of the window as the delayed ACKs trickle in. All 40 segments arrive twice and throughput collapses even though the network lost nothing.