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TCP Retransmission: RTO, Fast Retransmit & RACK

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TCP's reliability guarantee

TCP guarantees reliable, in-order delivery even over unreliable networks. When packets are lost, TCP must detect the loss and retransmit. The challenge: the sender doesn't know if a segment was lost or just delayed — and over-aggressive retransmission wastes bandwidth.

TCP has evolved three loss detection mechanisms: the conservative RTO timer, the faster duplicate ACK / fast retransmit, and the modern RACK time-based algorithm.

Packets in motion — click to inspect:

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