TCP timeout

TCP uses a timeout and retransmission mechanism. On what measurement does TCP base its timeout value to decide when to retransmit a segment?

Question

TCP timeout

TCP uses a timeout and retransmission mechanism. On what measurement does TCP base its timeout value to decide when to retransmit a segment?

BlackTom AI · Accepted Answer

Question restatement: TCP timeout. TCP uses a timeout and retransmission mechanism. On what measurement does TCP base its timeout value to decide when to retransmit a segment?

Option 1: The estimated bandwidth of the connection. This is not what TCP uses to timeouts; bandwidth estimation helps with pacing and congestion control in some algorithms, but the timeout is not derived from bandwidth alone. Misconception: assuming a link with higher bandwidth changes when to retransmit directly, ignoring propagation and queuing delays.

Option 2: The number of segments sent. The count of segments in flight does not determine the timeout. This would imply a simplistic, count-based retransmission policy, which does not adapt to network delay variations. Misconception: quantity of packets sent does not reflect current round-trip conditions.

Option 3: The size of the congestion window. The congestion window governs how much data can be in flight and affects throughput, but the timeout mechanism itself does not set timeouts directly from window size. Misconception: conflating congestion control state with timeout timing; while related, the timeout relies on timing measurements, not just window size.

Option 4: The estimated round-trip time (RTT) of the connection. This is the measurement used to set the retransmission timeout. Conceptual link: TCP measures the time for a segment to travel to the receiver and back (RTT) and uses that estimate, with some variance, to schedule retransmissions and avoid premature or excessive retransmissions. Correct idea: RTT estimation (and its variance) underpins the dynamic timeout value, as in algorithms like Karn–Matthews and Jacobson’s RTT estimators.

Additional notes: Some descriptions of TCP timeouts introduce related metrics such as RTT variance (RTTVAR) and smoothed RTT (SRTT), which further refine the timeout calculation, but the core measurement is the RTT estimate itself. The other options describe aspects of TCP behavior (bandwidth, segments count, and congestion window) that influence performance or flow control but do not define how the retransmission timeout is determined.

TCP timeout TCP uses a timeout and retransmission mechanism. On what measurement does TCP base its timeout value to decide when to retransmit a segment?单项选择题

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