TCP Retransmission

Duration: 4 min

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  22. Paper 2 | Distributed System: Distributed Systems
  23. UPPSC Live Class Recordings: UPPSC Polytechnic Lecturer 2025 (CS)
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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This lecture explains TCP retransmission, focusing on how a sender recovers from lost segments. The instructor first introduces the two mechanisms that trigger retransmission: timeout timer expiry and receipt of three duplicate acknowledgements. The first part details the timeout process, where a sender starts a timer for each segment; if an ACK is not received before the timer expires, the same segment is retransmitted. The second part introduces fast retransmission using a worked example of five segments where segment 2 is lost. The receiver sends an original ACK for segment 1, then duplicate ACKs upon receiving segments 3, 4, and 5. After the sender receives three duplicate ACKs requesting segment 2, it retransmits segment 2 immediately without waiting for the timer to expire. This is termed early or fast retransmission.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The lecture begins with a slide titled 'TCP Retransmission,' explaining that a lost segment causes the receiver to send an acknowledgement with the same ACK number. The instructor outlines two triggers for retransmission: 'Either Time Out Timer expire or it receives three duplicate acknowledgements.' The slide then transitions to 'Retransmission after Time out Timer Expiry,' showing a Sender-Receiver diagram. A red X marks the 'Lost Segment' before it reaches the receiver, while a timer icon tracks from 'Timer Starts' to 'Timer Expires.' A blue arrow labeled 'Retransmit Same Segment' illustrates the sender's action after expiry.

  2. 2:00 – 4:13 02:00-04:13

    The instructor presents a worked example where 'Sender sends 5 TCP segments' and the second segment is lost. The receiver's behavior is detailed: it sends an original ACK for segment 1, followed by the '1st duplicate ACK' upon receiving segment 3. As segments 4 and 5 arrive, the receiver sends the '2nd' and '3rd duplicate ACKs,' all asking for segment 2. The diagram shows a red X over 'Segment 2' and labels reading '3 duplicate acks received.' The instructor concludes that the sender retransmits segment 2 without waiting for its timer to go off, a process identified as 'early retransmission or Fast retransmission.'

The lesson progresses from general TCP recovery principles to a specific fast retransmission scenario. The core concept is that TCP uses two distinct methods to handle packet loss: a passive timeout mechanism and an active fast retransmission triggered by duplicate ACKs. The worked example of five segments effectively demonstrates how the receiver's inability to process out-of-order data (segments 3-5) generates duplicate ACKs. This allows the sender to infer loss and act faster than a timeout would permit, which is central to maintaining efficient data flow.

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