How does TCP detect segment loss?

TCP detects segment loss in two main ways:

1. by a timeout,
2. by repeated acknowledgments (duplicate ACK).

More simply: if an acknowledgment does not arrive in time, or the receiver reports the same byte several times, TCP considers the segment lost.

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#Method 1. Timeout (Retransmission Timeout, RTO)

#How it works

• TCP sends a segment and starts a timer
• if the ACK does not arrive within the expected time:
  • the segment is considered lost
  • TCP sends it again

#Characteristics

• it always triggers, even if the network "stays silent"
• it is slower, because it has to wait for the timeout
• it is reliable, as a "last line of defense"

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#Method 2. Duplicate ACKs (Duplicate ACK → Fast Retransmit)

#How it works

• TCP expects data in order
• if the receiver gets segments after a gap, it:
  • acknowledges the same ACK number several times
• when the sender receives 3 identical ACKs in a row:
  • it concludes that the segment between them is lost
  • it immediately retransmits it (without waiting for the timeout)

#Why this is fast

• there is no need to wait for the timer to expire
• the loss is detected from the receiver's behavior
• it significantly speeds up recovery of the transmission

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#A visual example

1. Segments are sent: 1, 2, 3, 4
2. Segment 2 is lost
3. The receiver gets 3 and 4, but is waiting for 2
4. It sends ACK = 2 several times
5. The sender sees 3 identical ACKs
6. It performs a Fast Retransmit of segment 2

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#What is important to remember

• TCP never gets an explicit "packet lost" message
• it draws conclusions indirectly:
  • from timing
  • from repeated ACKs
• after a loss, TCP:
  • reduces its transmission speed
  • tries not to overload the network

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#In short, for the interview

TCP detects segment loss by a timeout on a missing acknowledgment and by repeated ACKs (the Fast Retransmit mechanism), which allows lost data to be retransmitted in time.