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Networking Fundamentals · Intermediate

Retransmission

Resending TCP data that was not acknowledged within a timeout or was reported missing by duplicate ACKs.

Definition

TCP uses a retransmission timeout computed from measured round-trip times and fast retransmit after three duplicate ACKs. Frequent retransmissions in a capture indicate packet loss, congestion or a faulty link. UDP has no retransmission, so lost UDP datagrams are simply gone unless the application resends.

Example

Wireshark flags "TCP Retransmission" when the same sequence number is sent again.

Exam tip

High retransmission counts point to loss; check interface errors, duplex and congestion.

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