Suggest an editImprove this articleRefine the answer for “How does TCP detect segment loss?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**TCP detects segment loss in two main ways**: by a **timeout** and by **repeated acknowledgments (duplicate ACK)**. **Key point:** if an acknowledgment does not arrive in time, or the receiver reports the same byte several times, TCP considers the segment lost.Shown above the full answer for quick recall.Answer (EN)Image**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**. --- ## 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" --- ## 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 --- ## 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` --- ## 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 --- ## 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.**For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.